I am striking out on a search for my subject here. NASA's Maptis, MatWeb, Google Scholar, and the ASM handbooks, among other places I've checked, have not zeroed in on this. Any ideas? Thanks.
I don't think any material will creep under hydrostatic pressure, unless it has a lot of voids or other discontinuities that generate shear stresses. Pure hydrostatic pressure generates no shear stresses.
You will most likely need to run the test yourself or have a test lab run it for you. Rubber materials vary, sometimes dramatically.
I agree with the previous posts that unless there are voids in your material, I doubt you will see much.
In my previous job we would run a hydrodynamic test (volumetric test) to obtain data for material modeling for FEA in place of assuming a poisson's ratio.