Exactly. Add +20 to the fighter's maximum HP when he dons the armor
simple enough for you?
I would also give a -1 to hit with the plate, because let's face it, you ARE more clumsy with it on. -2 if wearing the armor untrained. Since +ses and -ses are very hard to come by in this edition, it would be a huge disadvantage for low-str people to wear plate if not trained properly, and not only that even with training, should be trading a bit of offense potential for defense. I'd rather be able to kill an enemy 5% slower than 5 enemies get to kill me 20% faster.
EDIT : wait, not really sure that'd work with healing..sigh
Maybe a simple DR mechanic would be best. I'd trade -1 to hit per DR, mitigated by 1 for training, 2 for training + expertise (theme bonus at a certain level). so a guy in splint mail, with DR 2, training in medium armor and with the fighter theme for heavy armor fighting, would have no penalty...but a full plate with DR 5 would be insanely slow. Magic would reduce the penalty by 1 per +, so a fighter, at say, level 5 or so, with "Armor training 1", would have like -2 to hit. He'd have to be maybe 9th level to fight in full plate +1 with no to-hit penalty.
I'm comfortable with some kobold NEVER being able to hurt me with his dagger, after I spent 1500gp and a month waiting for my iron man suit. the little buggers would have to get their ogre friend to come give me a few whacks, or push me into a pit or a pool of acid. Bad, bad little doggies