Started testing and everything works as expected so far. Does retain the "unloads a none if assigned an ammo type and unloaded after recharging from empty" behavior if you attempt that:

 DEBUG    : cannot determine reload cost as none is neither ammo or magazine

 FUNCTION : item_reload_cost
 FILE     : C:\Users\Vincent\Downloads\Cataclysm-BN-relics-4\Cataclysm-BN-relics-4\src\player.cpp
 LINE     : 1315
 VERSION  : BN Please install `git` to generate VERSION, or set the variable manually

However, this is status quo behavior, artifact recharging will already do that if abused in this manner, so fixing it's most likely outside scope.

Untestable, but only modders can be affected.
Not a good practice, but it doesn't look like it will cause harm if merged. Other than modders being misled into thinking this is properly tested and can be trusted. Modders who don't use it won't see it.

Bit belatedly, I did some basic testing in the process of implementing chaosvolt/cdda-arcana-mod#307, compiles and recharging seems to work as intended. Tested both time-based and trap-removal types.

Will be able to also test solar when I get the opportunity to prepare an update for Cata++ as well.

I tested it manually by enabling TEST_DATA mod for regular world and spawning all the relics added here, then checking how they recharge (they're in TEST_DATA so they don't pollute translation files). This is in no way an alternative to automated testing, of course, but after artifact->relic migration they'll be at least testable by regular players.