The idea would be to enhance the 'pretend' option to do all the compilation process without really merging anything instead of just do a list of package that would be installed.
Uhhh this would only work with --nodeps (as in a chain of packages would require linking to objects not yet merged to ${ROOT}. As it is if you want this functionality I would recommend the 'ebuild' tool for development/debugging purposes as it will let you do this. Emerge itself is quite high level and I don't think this request is useful or any better than using the ebuild tool.
You can do this with individual packages using -B, but it's not really feasible with a list of packages. Later packages wouldn't have the packages earlier in the list available to build against. The later compiles would very likely fail.
Ok, this was just an idea :).. And how about to have a gentoo's install in some folder and chroot it after?
