ok tested. surprisingly the scaling is real good: not perfectly readable, but very fast. i suppose it could be useful sometimes.
however, if the RTG resolution is whatever 16 bit, in windowed mode only, the colours are greenish-garbled like it happened for saturday betas and filters. if you swap to RTG 32 bit everything is fine.
[edit] however as the scaling is possible for fixed proportional resolutions only, there should be the same menu for them as for the fullscreen, whereas in non rtg mode you need the sizes to be freely entered without regard to the proportions...
there should be the same menu as in the fullscreen, then, with a "custom" entry and a field right of it.
[edit 2 ] forgot to mention: òaunching whdload (or whatever non RTG application) from a RTG environment may cause the non RTG screen to be strangely resized if the windowed winuae resolution is larger than the one of the picasso mode