Often during emerging for ROOT=/prefix/... the same package is first build for ROOT=/ and then again for ROOT=/prefix/... It would be nice if portage could reuse the previously build binpkgs.
* The build for ROOT=/ happens during the same run of emerge. Hence no dependency recalculation happens in between.
Possible solutions (alternatives):
* Know in advance that a binpkg will be build during a certain step and consider this for the following steps.
* Search in each step for binpkgs before starting the actual merge.
* ... your idea ...