I'd thought I'd post back, as I have accomplished what I was trying so hard to do earlier. Quick recap; I was aimed to get a Passive Ability to fire off its Buff once, and only once, and here is what I've discovered:
One
Passive ability cannot do this, you will need two. Basically, one which is your One Shot Buff, which will fire off as soon as the game is started. The other is your disabler. So you have your first Ability set up and ready to go, but it stays on, even after the Finish Condition due to the fact that its Passive. So, now you will need to create another
Passive Ability, that
'AppliesBuffToSelf' and has the only Buff effect of
'DisableAbilities', with no Finish Condition.
This is where it gets clever

Although these Abilities are all
Passive, they seem not to fire off at
exactly the same time, or else this wouldn't work. On your Ship/Planet/Whatever add these two abilities,
with your main One Shot ability higher in the list than the Disabler. The first ability fires off, then is instantly disabled by the second, which is now also disabled. There will be complications if the unit you are trying this with has other Abilities, but I recall references to 'UltimateAbilities' and the Disabling of them.
I hope this helps someone out there, it helped me hugely, and will be included in Sins Plus 1.3