Actually NiCD's are in fairly heavy use for sailplanes. And most ARF sailplane designs still allow for their use.

In the "get real" department I would suggest that you remember that Nano-Phosphate cells are the current "up and coming" technology, and they have heavier weights than LiPo's.

Since adding a bit of weight is easy, be it with bigger packs, etc.... better that the plane be designed around this type of thing, and let the end user adjust as needed.