I swear I was once told by Diesieben there was a way to access the TE and there by the ItemStacks containing it (hasn't had to use the IBakedModel system yet).
Edit: I thought of a way it could be possible. But by god it might be a pain to implement. You could make the blockstate hold the x, y, z positions, of course this might cause a problem of having way to many possible combinations in the blockstate, but never fear if applying a custom state mapper and overriding the createBlockState method to return a custom BlocKStateContainer that doesn't load a blockstate or at least throws out those Properties, that problem can be averted. And then you can access the TE from Minecraft.getMinecraft().theWorld.getTileEntity(blockPos from state) and access the inventory. Though if I remember what Diesieben said correctly then this probably is not what he was talking about.