Well, it would seem that my decision to release JCE under a new licence was premature, and was done without proper research.
For a start, Creative Comons licences are not appropriate for software, and are incompatible with GPL licences.
A commercial variant of JCE would be okay if TinyMCE was the only library included, as this is released unde the LGPL licence, which makes exceptions for commercial software. Other included libraries and scripts (phpThumb, getId3 and the webfx Selectable Elements script) are released under GPL, which complicates everything, as the licence requires JCE to also be released under GPL or a similar compatible licence.
This sort of sucks.
So, I guess my options are:
Continue to release JCE as GPL or LGPL, but slow down the development. The amount of time and effort put into JCE with no financial return is becoming less and less appealing.
Provide two identical versions of JCE, both licenced under GPL, but one for sale and one for free, in the hope that commercial users of JCE would 'buy' the one.