I'm pretty sure the only item that needs special coding will be Periapt of Health. There are two ways of doing it that I can think of. The first would be to remove bonus health above the maximum allowed by the race and the other is to remove 50 hp down to a minimum of 1 hp. The latter would definitely be more powerful. Either way, you may want to compensate for any health lost thus far from the round.

Examples
Method 1
Max human with PoH was shot once and has 180 HP. He is hit by the Alchemist's ward. His health drops to 150. When the debuff wears off he would either go back to 200 HP or 180 HP, depending on if you compensate.

Method 2
Player with PoH is nearly dead and has 20 HP left. He is hit by the Alchemist's ward. His health drops to 1. When the debuff wears off he would either go back to 20 HP or 50 HP, depending on if you compensate.

When you finish with your planned items, I could see Heavy Boots of Armor, Life Preserver, and Stone of Health having the same issues.


I'll try to find two people so I can test out Equivalent Exchange tomorrow.

EDIT:

By the way, when you finish this class you may want to expedite Tome of Health. Especially if you decide to go with method 2.