IIRC, the AFI referenced applies only to the static callsign list (ex: AF1, RCH, EVAC, etc, i.e. callsigns good pretty much anywhere or for a specific purpose). For locals, it doesn't really matter what callsign you use, so long as there aren't duplicate callsigns flying in the same center at the same time. An LOA with the ARTCC helps deconflict all of the units flying locals within that center. That's what I did when I worked callsigns at UPT-added a bunch of local callsigns through coordination with centers and the other squadrons within the centers, and added a couple cross country callsigns to the static list following AFI guidance. It's a bit of a pain, but much less so than checking in with ATC only to discover a different unit/aircraft shares that callsign and if already on freq.