While booted into Catalina 10.15.7 running the following solved this for me:
sudo nvram IASICatalogURL=https://swscan.apple.com/content/catalogs/others/index-10.16seed-10.16-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog
I was then able to proceed with the RC2 installer.
ts85
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