120 wheel in a 130 spaced frame

I could get a longer axle but the hub shell width will still be the same so not sure what I’m gaining for the extra expense over the gentle persuading approach

If you're happy with flexing the frame in then nothing. I'd have a slight concern that the stays aren't flexing in evenly, leaving you with an asymmetric rear end (ooh missus etc.) Depending on the hub you could add spacers inside the locknuts, since you don't necessarly need the axle to locate very far into the dropout. Alternatively you could put a chain tug on the inside each of the track-ends, especially if you use ones with lugs that fit neatly into the track end itself?