It would be nice to NOT need to build Amanda binaries if you want to use a different location for the GNUTAR binary. Having a default location is good, but having a way to override that (for instance, providing a known-good tar binary that Amanda will use even if a distribution updates the default system tar binary) would simplify configuration management ("yes, it is the 'standard' Amanda package we use everywhere") even when some things need to be overridden.
In the case of some turn-key scientific instrument control systems, the vendors frown on modifying their system configurations, and it would be nice to still be able to drop the standard Amanda package on the system and not worry about using some old tar version causing grief.
I could see using an entry in the amanda.conf file, a command line option (I guess you'd need for each program that uses tar now), or maybe an AMANDA_GNUTAR_PATH environment variable. In any case, specifying where to get the tar binary without rebuilding the amanda binaries is the goal.
ps: I'm assuming this is the right place to post "requests for enhancement". If not, please excuse this posting, and move or remove it as appropriate.