Hm okay, so I just finished testing it. It definitely works, so to be clear this is what I was considering:
3 Effects on the ability:
Heal over Time
The players I had using this setup would use it as an attack. I had the Heal over time cancel out the damage from the Siphon, then had the “Damage” effect deal less than the Siphon. This way the only net damage is the “Damage” effect. And as long as the Siphon is higher, the player was healing more damage than they dealt.
The only issue is that this does still have nuking potential. Since the Heal over time happens last, the initial damage is both the “Damage” and Siphon added together, even if one is counteracted later.
Edit: There’s actually a much bigger issue. “Protect” effects on monsters causes the damage to be ignored but the Heal Over Time to go through normally, meaning if a monster is using a “Protect” ability, they net more health than the player does from the attack.