That's mostly meant for distribution installation or recovery (when your active root fs isn't the same as the root fs of the distribution you're maintaining). It isn't that great for instaliing a package to arbitrary locations and expecting it to work without lots of manual fiddling afterwards.

RPM also supports the notion of "relocatable packages" which can do what the OP wants (provided the packager spent a little bit of extra effort to make the packages relocatable), but that's not going to help until "real" MeeGo comes along at least.