Sounds great, but that's only an idea. Basically you have to make a new ebuild for new versions in gentoo too, so this builder system is not bad on this part.
If you think that a package should be a better version, then edit its rules and see if the package builds with it. In some cases it does not work. So you have to edit it more. In some cases it works, but it makes other parts of the system not work/not compile.
I guess that's why Sashz doesn't update everything in a release (and why Debian has so damn old packages when it's finally released). But if you edit a rule and it seems to be cleanly built, send the patch/edited files to Sashz, I'm sure that he will look into it if it's stable enough to build the system with it.
Zumi