We have sort of a ladder for our nursing assistance -- but it only has 2 levels. The "advanced" CNA's serve as preceptors and help work on policies, inservices, etc. that directly involved the CNA's.
I would think you could use the same principles to create an LPN ladder as you would any other ladder. For example, you could base it on Benner's "novice to expert" framework, like a lot of RN ladders are based ... or you could base it on the assumption of leadership functions within the LPN group, as our enhanced role for CNA's is.