After a lot of trial and error, I figured out how to add hit-dice to a snippet (so you can write, for example, spend a Hit Dice to regain 1d6 + 3 hitpoints). I figured I'd share in case anyone else was looking for a way to add this. This only works for single-class characters (or multi-class that share the same hit die) that don't roll their hit dice.

The expression is: {{(modifier:con*characterlevel+characterlevel-maxhp-1)*(2-4)/(characterlevel + 1)#unsigned}}

If you're curious about how I got this expression, 

maxHP = (hitDice + conModifier) + (characterLevel - 1) * ((hitDice / 2) + 1 + conModifier), solve for hitDice. It's (2-4) and not (-2) because it throws a NaN error when you start an expression with a negative number.

thank you!  exactly what I needed for the dwarven racial feat i'm entering :) 

Dude, you're a wizard! I can't believe you figured this out. I've been looking for a way to do this to tie an ability activation to hit die in my custom subclass for a rogue I'm building. In this vein, is there any way to get the little clickable box in the ability's description using hit die? Similar to the box for spell slots, or special activations of an ability a la the Blessing 
of the Raven Queen ability on Shadar-kai elves.