[thejeff]

No, it's based on the unit's id number.

Still, since you know it, you can predict it. In your case since only the masters need cast Air Power, just have the first master in the list cast Storm.

Ideal order for this combo would have all the Jarl slaves first, then the Storm casting Drott, then any other Drotts, who cast Air Power.
The Jarls get Air Power from the Masters, the Drotts are boosted by their own air power and the communion. Since the Jarls act before any Masters they will cast on their own, letting you get off thunderstrikes.

If the jarl slaves are after even one Master, they won't cast and you'll just have a very expensive communion powering a few Drotts.

I'd still worry about any Drotts with paths other than Air and Blood. Once they go off script, with no more rejuv, casting anything offpath will kill off the Jarls quickly.