Thanks, Lyndel!
I have now found tried a "DotNet35Setup.exe" with a size of about 100 MB and a "dotnetfx35.exe" with 230 MB in size.
Both show a popup after launching "This app can't run on this device" and "This feature is no longer part of this version of Windows".

Strangely, in the umpteenth attempt I managed to get to the the actual installer dialog for the smaller of these files, but installation fails for a couple of DLL files because of missing privileges (despite running as administrator).

I don't want to spend any more time with this, maybe there's a way, maybe there isn't.
I'll stick with the "new" VM for now.