If you silver plate to 5 skin depths, then the base conductor properties should have negligible effect on microwaves. Depth of penetration means just that, for magnetic as well as electric fields.

Somewhere on this site, there is an analysis of thin gold over nickel plating, which shows that this is rather bad for microwave losses, as the RF penetrates the gold into the lossy nickel. I am sorry, but I cannot find the page right now. For much thicker silver plating over pretty much any base conductor, then it appears that one can ignore the base conductor for microwave performance.

I note that copper or silver coated steel conductors are often used in fairly high grade coax, to provide additional strength and mechanical stability, without degrading RF losses. The only case I have seen where copper coated steel conductors present a problem is cheap cat5 cable used for power over ethernet (POE), where the DC volt drop is much worse than pure copper conductors.

Thanks Glyn :)
It makes sense that both E and H component stays within the same boundary, and if the silver is thick enough it should be fine.
Yes, I found this document covering the potential problem with too thin gold over nickel:
https://www.simberian.com/AppNotes/ModelingConductorLoss_2016_01.pdf
I guess it also applies to waveguides and cavities for mw.