I was thinking about creating a custom character that could create magic items without being a spellcaster. I note that the construct-creating powers of a Dwarven Machinist counts as three skills. Would it make sense to treat magic item creation as an ability of similar magnitude?
You might be able to work backwards from the magical source creation rules in Axioms. It talks about minimum ratings in certain forms of magic to gain access to things that mages and clerics have by default. That being said, that's still intrinsicly tied to casting spells, and most magic items are built around the idea of getting more regular use out of existing spells. I don't really have a better suggestion than that.
Both Alchemy and Dwarven Machinery were rated as worth 3 proficiencies for non-casters. Beyond that, there are no hard and fast rules for this. My vaguely-formed intent was that each type of magic item or field of magic research would require a different set of proficiencies/class powers. So weapons/armor, wands/staffs/rods, miscellanous magic, and scrolls would each have their own proficiency/class power, as would constructs, necromancy, ritual magic, etc.
You could build from the system used for Alchemy. So you could imagine an "Enchantment" proficiency which enables the creation of miscellaneous magical items, where 2 slots allows you to work as an assistant and 3 slots enable you to create items if you have a formula or sample.
You could also build from the system used for Dwarven Machinist and make a class around it. For instance, I could see a class which can scribe scrolls and later acquires the ability to do ritual magic, and is essentially a sort of ritual/ceremonial wizard type.
Yes, a specialist class was what I had in mind. It's possible that our custom setting will have Dwarves without any casting ability at all, in which case I want a Dwarf custom class that can enchant items.