The problem with using it to dust your insects, is that there isn't a general guideline for how much spirulina a cham needs, or can process safely. So the frequency of dustings would be tricky to work out.
That said, I have heard of spirulina being recommended as a way to deal with fussy chams who reject certain feeders. It has often been stated that chameleons have a preference for green feeder insects. So by dusting tricky feeders with the green algae powder, it supposedly makes them look more appetising to the chameleon. I've tried it, but I can't say it really made any difference...
So in summary, I'm sure you could use spirulina to dust your insects, as long as you don't allow it to disrupt your usual supplementation schedule (i.e. only use it for additional dustings, don't let it replace your usual calcium wo/D3-calcium w/D3-multivitamin routine) and don't mix it up with the calcium (rather use them at separate dustings). Though I don't know how much more beneficial it would be than simply using it as a regular part of your gutload...
