Conclusion/Summary
I actually think that this is not in the first place about the positive/negative or concrete/abstract dialectics Hegel later will dive into, it is a affront against Kant. Kant's ethics in general could be read as purely negative, though. 'Kant may have presented us the grapes', as Goethe once said, 'but rejected us the way to it'.
The highest form of human excellence is accessability of the ideas in form of a positive determination through intellectual intuition, one might say. And the task of the philosopher is not only to perform it and express the insights gained, but to make it accessable to others through poetry. This is the core of Hölderlin's own systematic take on philosophy as I understand it.
Argumentation
I will argue here with the Oldest System Programme of German Idealism from 1797, which, although written in Hegel's handwriting, is obviously heavily based on Hölderlin's thinking of that time, who arguably has been the only one able to include all of the kantian, fichtean, spinozian and aesthetical aspects included. I am well aware, though, that since Rosenzweig most people tend to Schelling's authorship. It actually is still an open debate going on, recently once again inflamed by Förster.
Kant, as positing reason and intellect in the very center of his philosophy, developed an ethical system that rests on the foundation of the Categorical Imperative, a principle (or idea), that in nuce only excludes acts as inproper/injust. His ethics is adressed as follows:
Since all metaphysics will henceforth fall into morals-- for which Kant,
with both of his practical postulates has given only an example and exhausted nothing, so this
ethics will contain nothing other than a complete system of all ideas, or what is the same, of
all practical postulates. (Oldest Sytem Programme of German Idealism)
And it continues:
From nature I come to man's works. The idea of the human race first-- I want to show
that there is no idea of the state [Staat, i.e. governmental structure] because the state is something mechanical, just as little as
there is an idea of a machine.
Only that which is the object of freedom is called idea. We must therefore go beyond
the state!-- Because every state must treat free human beings like mechanical works; and it
should not do that; therefore it should cease. You see for yourself that here all the ideas, that
of eternal peace, etc., are merely subordinate ideas of a higher idea. (Ibid., bolded by me)
My interpretation would be that here there lies the main line of attack against kantian ethics: Justifying governmental structures as rational violates the very idea of free beings as they are only treated as mechanical elements there. Hence the only way to do philosophy proper is to go beyond that:
Finally the idea which unites all, the idea of beauty, the word taken in the higher
taken in the higher platonic sense. I am convinced that the highest act of reason, which, in
that it comprises all ideas, is an aesthetic act, and that truth and goodness are united like
sisters only in beauty-- The philosopher must possess just as much aesthetic power as the
poet. The people without aesthetic sense are our philosophers of the letter. (Ibid., bolded by me)
As you might see, kantian ideas are not in the higher platonic sense, i.e. being able to be intuited [angeschaut] and holding a deeper truth. The highest act of reason is not intellect in the sense of the Hyperion, as I take it, since it is an aesthetic act. Kant would be philosophers of the letter for the most part.
The main point behind it is not to remain in (rationally judging) intellect and actually prevent ideas from coming into existence, but to build a bridge under the name of the idea of beauty, which may contain both rationality and mythology:
Until we make ideas aesthetic, i.e., mythological, they hold no interest for the people,
and conversely, before mythology is reasonable, the philosopher must be ashamed of it. Thus
finally the enlightened and unenlightened must shake hands; mythology must become
philosophical, and the people reasonable, and philosophy must become mythological in order
to make philosophy sensual. (Ibid.)
Therefore, Kant (and Fichte, Hölderlins teacher 1794/95!) fell short of the actual task of the philosopher, although Kant in particular nailed the ideas and Fichte introduced intellectual intuition as actual possibility (even necessity!). Kant did not manage to flesh out (or fully embrace) the highest idea, beauty, and denied intellectual intuition (although he touched both in the third critique). Hölderlin clearly bases his philosophy on the latter (as seen in Judgement and Being).
Arguably, the Phenomenology of Spirit by Hegel did follow the same task of 'uniting the unenlightened and enlightened' according to the preface, but fell short of the poetric (and/or mythological) part as well, as every reader might readily confirm.
To explicetely adress justice
I think the idea of justice is adressed as well:
(The) absolute freedom of all spirits who carry the intellectual world within themselves,
and may not seek either God or immortality outside of themselves.
The problem is: It is but one part to nail what makes this impossible (Kant, Categorical Imperative), but it is a totally different task to actually provide a positive way of its realisation. I think the overcoming of the mythology/philosophy divide through the 'highest act of reason' is actually proposed as a positive way and corresponds to 'the highest form of human excellence' mentioned in the opening quote.
Aside
Kant would actually not opposed this, I think. It simply was not his task. He only wanted to 'nail the ideas', as I called it. He even wrote himself that there are means through which morals do come into actual application (e.g. §13 of his Anthropology).
