I asked some time ago about the possibility of building experimental versions of packages, such as Seamonkey 2.1 alpha or beta, and installing to a nonstandard location such as /usr/local2 or /usr/testing. Then I could run such a package by spawning a subshell with /usr/local2/bin and /usr/local2/sbin at the start of the PATH, Check out the guile14 package for an example of not exactly what you want.
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