VG,
I like your thought process. You're right on target about 'like ablative but fixed more quickly'. The hard part, as with most things in this system, is making the sometimes arbitrary determination of the value of a limitation. Also common with Hero is the fact that there are many ways to do the same thing depending on how strictly you want to adhere to 'official' guidelines and what specific game mechanics you're really looking for. As an example, I know of a mentalist NPC that the GM bought the Armor power for, with the justification that due to his mental abilities 'people just don't like to hurt him'. In my mind that's a bit of a stretch based on the description of Armor. I might have tried something much more mechanically complicated like Uncontrolled, triggered suppress that affects attack powers when attacked. That could be extremely expensive much like the healing in your analysis.
Anyway, I'm inclined to agree after further thought that the healing idea is too expensive. I think the limitation I'm looking for is not worth as much as ablative because it can be restored more quickly, but not too much less than ablative because restoring it causes damage.
I guess that leaves me welcoming any other ideas people can throw out.
Thanks!