It would be very nice indeed!
Where is the source?
I expect that whilst compiling OWB for OS3.x is perfectly possible* (WebKit itself is pretty clean, you'd probably need to spend most time on the GUI side), the end user experience is likely to be a bit disappointing without a great deal of effort put into optimisation. And by that I don't mean changing a few, frequently called functions for inline assembler variants (as fun as it is), but a very detailed analysis of the overall flowgraph for the layout engine / renderer to see if any of the algorithms need to be changed.
*Caveat: the core WebKit code is largely written in C++ and makes a lot of use of abstractions and generics. It may prove difficult to compile for existing amiga m68K targets.