Depends on what you mean by passive. Like an aura that affects all nearby enemies or a random enemy every N seconds? The unit with the ability acts as if parasite was cast on it all the time?
The aura wouldn't be too hard. Just make a dummy aura that affects enemies, and when a unti dies, check if it has the buff the aura gives, and if so, make a watery minion thing. If you want it to act just like a summoned unit, you could make a dummy caster with a regular summon ability (like summon bear). For random enemies nearby, every N seconds pick a random enemy nearby that doesn't have the Parasite buff, make a dummy caster, and have it cast parasite on that unit.
If you want the unit to just spawn a minion on death, go ahead and do that with a trigger.
I don't think I'm dealing with quite the same pressures though, at the very least its a significantly smaller system. For the time being I'm just going to put together a quick scrubby box though and hope it works good enough to not make my house toxic
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.