I would pick up the relay signal at the alternator. If it isn't charging then the HID is not active. Would solve the problem for guys with the plasma lights too, once they fire if there is a significant voltage drop(starter motor) they will not restart.
If you think about it,
This is a not good point for the 12V source for the HID ballast.
The output lead of an alternator goes direct to the battery all the time, key on, or key off. It only outputs power when the engine is running, that is true, but the lead itself always has battery power on it. ( reverse blocking diodes are inside the alternator )
On my bike, DaveO430's bike, and Lopeha's bike, Dave used the oil pressure switch signal to ground as the switching point. The delay relay is "hot" all the time, you just interrupt the Ground Signal instead.
The idea is, you do not want the ballast to fire up unless the engine is running.
Another source for the Delay Relay would be the Accessory switch block. That is only on IF the key switch is turned on. But, this has a problem if you don't actually try to start the bike.... the HID will be turned on at the time out period anyway, regardless of whether the engine is running.
IMO, as an electronics background person of 50+ years, you want a switching point that is operated by Oil Pressure.
This negates all of the bad places to hookup the power. You get the 12V + direct from the battery thru a fused wire ( close to battery ).
The switch lead is the ground signal from the oil pressure switch.
You definitely do NOT want to source the +12V power lead from the OEM headlight switch, as that is pulsed with the START button....