The question is, would it be easier / faster to port Netsurf so that it doesn't use SDL, uses MUI and works fast. Or would it be easier / faster to update AWeb (which is open source, I believe...)
The former.
Is netsurf for OS4 SDL based, or is it reaction/mui?
OS4 NetSurf is Reaction-based. There shouldn't be anything there which wouldn't work on OS3.9u2, the actual GUI stuff is pretty simple windows/buttons/empty space/tabs with some dynamic adding/removing (which can always be stuck in "always there" mode if necessary). It may need some slight reworking to replace new OS4 functions with older ones.
The hardest part will be modifying the plotters to use an 8-bit palette, and the UTF-8 text display will need replacing (I have some old code using ttfengine which should work as a replacement with some updating).
It'll take somebody a few weeks to "back-port" it.
If anybody does this and bases it on the OS4 code, please just #ifdef so improvements benefit both versions.
Oh, and join the netsurf-dev mailing list!
Chris