When you say 'costing codes', do you mean code combinations? If so, I think that would be impractical to be honest, given the likely number of combinations of costing segments. Also, the fact that an assignment can have split costing could make for some interesting challenges!
I believe you can already secure by payroll and org (which are costing levels in themselves) and of course by position (which to my knowledge is not a costable entity)
_________________________
L&K
CT
Remember: A dog is for life, not just for Christmas... unless you're in Korea