"Not sure that we have any precedent for dealing with tests that require admin privileges when installed but not in the source tree?"

I think we do.  With the OS X installer (and, in theory, other Posix-style installs), we have a similar situation where tests are installed without a source tree and I believe they should be expected to be runnable by the user without admin privileges, where necessary, skipping those few test cases that *might* actually need elevated privs.  Granted, we currently don't do a lot of systematic testing of tests from installed locations.  I do as part of the OS X installer building and testing.