Using an ability uses a Hit Die

[Fenris447]

One of the things not yet programmed is the release version of the Aberrant Dragonmark. I have a decent amount of it covered, but my first snag is how it allows you to choose to spend a Hit Die. Here's the details

1. You have a spell you can cast 1/short rest
2. When you cast the spell, you can choose to also spend a Hit Die
3. The hit die either gives you a small temporary HP pool or deals damage to someone

So programming #1 on its own is pretty easy. Adding the temp HP pool in #3 is also easy enough, except we'll have to have the user indicate whether they got the pool or they did the damage, as it's based on the die roll.

The real tricky part is #2. We need to be able to deduct one HD from the overall pool (which will probably be even more complicated if they have multiple HD pools from multiclassing). It can't be an activated ability that just reduces the HD tracker (which I also don't know how to get at) by 1, since multiple hit dice can be used for this between long rests. It basically has to be something that, when clicked, "permanently" reduces the HD tracker, until that tracker is added to as part of a regular long rest.

So I guess what I'm asking is this:

• Can an effect be instantaneous (IE not a constantly-running eval script or bootstrap)?
• Does anyone know how to tell Hero Lab to deduct one Hit Die?
• How do we combine the answers to these questions?