In the unit properties tab, there's an unlabeled number box that I haven't been able to figure out what it does. It looks like this:

Hitpoints [150 ] [0]

Does that [0] do anything interesting?

It's the final, hidden byte of hitpoints; it's typically edited to create special wireframes (e.g. dark blue marine wireframes), I believe StarCraft internally uses it for hp regeneration.

Huh. Might be color, since zerg don't have a difference, unless its something baked into the code. Probably not going to affect much to be worth messing with.

Also, there's no way to change how much max energy a unit has, is there?

Not without plugins (only compatible with 1.16.1)

Starcraft stores HP in an 256-multiplied form, so that they can deal with sub-integer hp modification like zerg healing or 0.5 damage.
Map stores HP in its original form, so that [0] is the last fraction of HP. Think it as a remainder of HP divided by 256.

In technical terms its 24.8 fixed point.