Wouldn't you have to suppose it's monism to maintain any universal theory?
Any epistemological theory (which I think you would need to posit and prove a metaphysical theory like this) relies, I believe, on universally consistent laws of reality. If the fundamental substance(s) of reality we're dulistic, pluralistic or a hegelian 'becoming,' then wouldn't this rule out any universal epistemological theories that you could you to posit these theories in the first place?
For instance, if it's dulistic, then we have Fundamental of Existence A and Fundamental of Existence B (I'll refer to them now as FOE A and FOE B.)
By definition FOE A and FOE B would have to be different in their essential nature's or they could simply be grouped together as having SOME universal property (therefore devolving back into monism.) If FOE A and B are different in their essential nature's, then it makes sense to me that you'd need two different epistemological theories to deal with each reality. These theories would then also (according to their metaphysical base) be different in their essential nature. But, if you have two epistemological theories, completely different in their essential nature, how could you know truth? If I propose dulism, and am consistent with it, which epistemological theory am I basing that knowledge off of, FOE A or B? And why is that one more valid than the other FOE?
It seems to me that you'd have two methodologies to 'truth' both equally valid, which devolved into, essentially, subjectivism, which can't claim knowledge which therefore invalidates the dulistic position. Pluralism seems to me refuted in the same fashion.
I believe you'd have to have some fundamental 'nature' or 'essence' to reality (monism) to make any sort of epistemological claim I believe Aristotle and Rand said something similar to this effect.
Becoming I believe also devolves into subjectivism, which Neitzsche pointed out thoroughly in The Will to Power, if I remember correctly.
Perhaps I'm way off the mark, but these were my thoughts, let me know what you think!