As far as other lower options; The high quality 'mil spec' Lewis Machine and Tool springs to mind. I hesitate a little to print an easy to digest 'list' of non-named AR/AK lowers, but I CAN list the ones that ARE banned, and by definition the ones you find should be legal (subject to DOJ approval, I would get a letter of your own and not rely totally on mine.)
Category 1 & 2 both in an easy-to-digest list: http://ag.ca.gov/firearms/regs/aw.htm
This includes the usual CPC 12276(a)-(c)
AND the elusive yet law; CPC 12276(e)
If it's not on there, it's worthy of a DOJ letter at the least.