Once per battle. A character can't gain more points than those lost in the battle.
That's basically a copy/paste job of an optional rule from Sword and Wizardry. You heal 1d6 hit points, but it can only performed once and you cannot regain more hit points then what you lost in that battle.
I know the idea of this is to remove the healing spells, so, until we have a rule for that, how about removing "cure serious wounds" and replace "cure minor wounds" with "cure wounds".
Cure Wounds would be a first level spell that has an incremental healing value (basically it is HSW lowered at 1st level.)
Maybe something like 1d6+1 per X levels. And, optionally, the GM can use the "held magic missile" optional rule which allows the caster to use part of the spell, so, instead of healing 3d6+3 the cleric can divide it up to 3 1d6+1 healings along the day.
This free the 4th level slots and, if the optional rule is used it also frees some 1st level slots.
Going even further the spell could be used at higher character levels to cure blindness, disease, etc. Using this could also mean removing the Heal spell (or using Heal instead of Cure Wounds) and freeing the 6th level slots.
Once per day, a character can have his wounds bind. Some bandages are needed, which cost 5 sp. If done in battle, a wisdom check is needed ; the bandages are lost if failed but it can be tried again. Clerics/druids heal a character 1d4 hp that way, 1d3 if the bandages are used by any other class.
So far, it doesn't seem game breaking and the players are liking it.
When using negative hit points, I like the rule in the BF Secondary Skill supplement. Heal - WIS check to stabilize character that is bleeding out. Or if the character has a background in first aid and role plays it well then prob just give it to them.