If you want a passive non-percentage ability then you'll have to do what SAUS said and use the ring of regeneration ability.
You may also be able to look at the Healing Ward, but I think it is based off of percentages too.
I'm pretty sure some of the aura abilities have a "Value as a percentage" field or something. If it is false, then it should do flat healing. I know that is the case with brilliance aura and thorns aura. I'm pretty sure I did it with a healing aura on my map now that I think about it.