I suspect it might work in the simplest sense~ Each point of mitigation/regen is worth a point of shield, you'd have some damage stored up to the regen value at any time.

This is my working theory:

Say you have 20 damage mitigation, and 80 regen.

While on full-life, you take a 150 damage hit. 150-20=130 is subtracted from your health bar. Of that 130, 80 goes to the regen value (shown in white)

Every turn you don't get damaged, 1/5 of 80 is added back to your health, so 16. If you get hit again, then the regen bar fills back up with the new damage.

Point for point, damage mitigation is almost always better than regen - but (1) you have access to a lot more regen, (2) a minimum damage always goes through mitigation. 10~20%, or so?