https://github.com/VazkiiMods/Quark/blob/master/src/main/java/vazkii/quark/content/client/module/UsageTickerModule.java#L200
https://github.com/VazkiiMods/Quark/blob/master/src/main/java/vazkii/quark/content/client/module/UsageTickerModule.java#L244

I have noticed that the code for the Usage Ticker that shows how many arrows you have left for bows and crossbows could probably be tweaked to detect any ProjectileWeaponItem cause it uses the generic methods to find its ammo.

Though I'll be honest the only mod I know for sure to use the vanilla ProjectileWeaponItem and Player.getProjectile (that isn't just extending the vanilla bow/crossbow classes, those get detected already) is Guns Without Roses (which got me a bug with a quiver mod that patched getProjectile incorrectly).

But hey could be worth a try.