One direct way of stating the heart of the question might be: is there sufficient textual evidence and support for the assertion that Plato's Republic contains meritocratic elements?
A second and larger concern here would be if so, because of this meritocracy, and not to mention the absence of slavery, the Republic could be considered in some sense democratic, at least perhaps moreso than the ostensible democracy of Greece, limited of course to land-owning males -- and perhaps in this context it might make sense to ask whether this "real" democratic element is present in our contemporary democracies.
In a way this question comes down to the following one: does the Republic include a meritocratic analysis of the population at every stage of psychic development? A possible equivalent today might be global universal education, which every child could gain entry to -- although higher levels would involve passing increasingly rigorous tests.
In other words, could the ostensible aristocratism betrayed by Plato's advocacy of the "noble lie" of the myth of the metals -- that certain people are gold, others silver, yet others bronze, etc. -- be read on the contrary as reflecting not a base classism (or worse a vulgar racism) but rather and perhaps counter-intuitively the very heart of the radically progressive or democratizing-meritocratic impulse that is the engine of the Republic?
Is there in the Republic a global selection over the population for traits that could be optimally deployed in various roles or functions within the society? What about the very negative authoritarian dimension of the Republic, the expulsion of poets and artists; as well as the de jure caste system which would seem to strongly favor an aristocratic interpretation rather than a democratic one?
I guess I want to graft the "democratic" onto a different chain of signification, and point to the inner radically democratic (because meritocratic) moment in the Republic where, at least from the perspective of a child, everything is possible: all possible paths are potentially open to everyone. What makes it "just" and "fair" is precisely this lack of socio-economic barriers to individual growth and development. I don't want to paper over the negative dimensions and palpable distaste for the "masses" that is present in Plato's text here; but does it seem at all justified to make this case for a possible democratic recuperation of the Republic?
Apologies for the length here. Just to reiterate the basic question: is there textual evidence for meritocratic social advancement and organization in Plato's Republic? (And if so -- as a secondary concern, that is really the motivation for the first question -- is it then reasonable to further characterize this aspect or impulse of Plato's thought as effectively democratic in some ways; might it be reasonable to say this meritocracy relates to what it means to have democracy?)