In Divine Divinity even just equipment bonuses to strength would help. Playing a warrior I was biased towards strength and agility (or constitution / hit points early in the game), and still ended up with enough intelligence and mana bonuses to have plenty of mana. I never played a mage, but I expect the opposite would be true, as well.
In Beyond Divinity there were two characters to split the loot between, and once you got your first summoning doll (relatively early in the game) you could have it carry an essentially unlimited amount of weight (when encumbered it wouldn't be able to move, but you could un-summon it and re-summon it elsewhere). The summoning dolls were creatures you could summon and control like your main characters (though not nearly as strong), who each had their own inventory.
In any case, it sounds like the character builds in Original Sin are expected to at least be competent with magic (4 of the 6 skill trees are magic and the ability to use chaos magic is the main part of the plot we know so far), so it is unlikely pure mages would be at a significant disadvantage even with a strength based inventory system.
