I can tune the viscosity of the resists that I spin, just by adding solvent. I guess the same thing could be done with photo emulsions... but my thought was simply to use temperature to tune the viscosity.

The flow rate for thin films isn't quite as you describe; in the thin-film limit, the interaction between substrate and wetting fluid really starts to matter. I don;t know offhand what the profile is, but the thickness profile is rather flat except near the edges. You can judge the film uniformity, directly, by watching color changes in the resist... it goes through a rainbow of colors as the film spins thinner, kind of like a soap-bubble effect.

What one does is work out some "spin curves".... film thickness as a function of speed, time, and solution viscosity. I have no idea what the ideal thickness of a photo emulsion is... a few microns? That'd be step 1, to try to get that right. Might require a few passes. If the solution is very viscous and there is a radial profile change, who knows, it might actually be interesting to see.