EDIT, so for future adventurers trying to do the same, I mimic'd Malenia's personal effect. She heals on hit but its different than our Bloodborne-esque Malenia effect.
What I did was make the healing effect. Make it 0 seconds and 0 interval so it procs once. Tick "Delete in Wait Mode" so it only procs on this moment and all at once. Make it a unique new StateInfo ID, obviously I am a man of culture so I went with "7777", and put it in the second slot, below "State Info", you actually want this to invoke the effect so it needs to be "Trigger On State Info". Whatever slot in that category works. This ensures the the effect only procs when you have that stateinfo active. This was all on the effect itself, the healing effect. Make sure it is tagged as "Trigger for Self" and target the usual targets, basically everything.
Now to "unlock" the effect so it procs, you will need to get the talisman you want it on, or the armor, or whatever way you want the effect to be active on you, and simply put this new effect on it that we will make. Make or copy the new effect, have it too ticked for self trigger, basically have it blank, have the effect itself have a duration of -1 as well as the interval. Have the State Info ID itself be 7777 this time rather than the trigger on stateinfo. This way the talisman gives the state info, which allows the effect to trigger. Not much to do on this effect, its more just a key to unlock the heal effect. You can put health regen or something on it if you want, but bare in mind the stateinfo is already taken and it has an infinite duration so. Do not tick "Delete in Wait Mode" for this one.
Now the somewhat dangerous part, I'd advise only mass editing actual projectiles you can guarantee are the player's, like arrows and stuff like that, else you'll end up with weirdness or potentially replacing important effects. Go to Bullet in the param editor and highlight all the bullets you want to heal (yes I know, it sucks) and just mass edit the 4th target effect slot to the healing effect that triggers when you have the talisman or whatever on. If you have the talisman which gives the effect which gives the stateinfo, these projectiles will heal you, if you don't have the effect, nothing will happen.