Would it be possible to do this similarly to Conary... only installing
the files (.so's and things in /etc and /usr/share/{icons,sounds,...}
etc) required by a given application (binary with .desktop file) ?
This would provide similar to package splitting, but because of version
control and something like google update, it can be effective to update
only those files and applications when its parts are updated upstream...
with something like presto only bringing in what changed, and something
like jigdo allowing you to download each file from a different mirror,
speeds can be quite quick and downloads quite small.