thanks for the replies everyone,
so I guess I've got my answer: there seems to be agreement that the ROM header cannot possibly address more than 1MB so, my idea of using it for large-scale flash storage (akin to a HDD) is out of the question. Oh well, there's always AdIDE for A500s I guess.
I'm personally not that interested in changing the kickstart itself (I'm amazed that as much effort has gone into dedicated projects to do so), as kickstart switching hardware is abundant and the chips themselves do not seem to be in sort supply. Even when considering custom kickstarts, such as the fabled 3.9, I cannot imagine much more than marginal improvements in performance by having some of the OS in ROM (some of which may be offset by comparatively slower access times on fast systems).
If a way could be found though to add cheap flash based storage to all amigas, then this is a different story. I bet that a full commercial run of AdIDE68k (and a CD32 version of it) would be a huge success.