The trick there would be the compiler. Both of them require fairly current ANSI C compilers -- Gecko won't build on less than gcc 4, for example, and I know WebKit really needs at least gcc 4.2.
Short of that, knowing what I know about the individual targets, WebKit would probably be more amenable (Uzbl comes to mind).
However, I have always thought that proxy approaches for old machines have been criminally underutilized. Such a proxy could even be written with native tools, such as MacPython or MacPerl. The proxy then does the "distillation" into something a native browser could understand. I used such a setup for the C64 when I was maintaining HyperLink 2.5.