BryonD
Hero
I don't think it makes sense to assume that healing happens suddenly at the 8 hour and zero minute mark. So it would be completely reasonable to "pause" the game for a moment if 1 hour+ into a long rest and allow the spending of HD. At the end of the long rest (assuming the fight didn't take an hour....
) the PCs regain their HP, but they might have less HD for having spent them during the night. This doesn't seem to conflict with either balance or narrative flow.
Because of the way my house rules work, this may not be helpful to most groups. but FWIW:
PCs only heal 10% (not 100%) with a long rest. So they are often going to spend HD for healing either way. Allowing this choice to be "achieved" after the first hour makes sense anyway. You can already spend HD one at a time and see the results before choosing to role another. So if you know you are going to spend 3, I'd give that at the hour mark. You could then decide to spend 4th or more at 1 hour, or the end of the full rest or whenever.

Because of the way my house rules work, this may not be helpful to most groups. but FWIW:
PCs only heal 10% (not 100%) with a long rest. So they are often going to spend HD for healing either way. Allowing this choice to be "achieved" after the first hour makes sense anyway. You can already spend HD one at a time and see the results before choosing to role another. So if you know you are going to spend 3, I'd give that at the hour mark. You could then decide to spend 4th or more at 1 hour, or the end of the full rest or whenever.