Was going to let this go, but had a couple of Jacks so here goes.
There has been a load of tosh spouted above.
The question was "is it legal?"
Surely the point is not whether or not some nerd in the rule department has published a rule about this but whether the professional aviators believe this is a sensible thing to be doing.
My comment about the headset has distracted from my point of concern: legal or not is it a sensible thing to be forced into doing.
Is it poor design not to include adequate sun filtering?
Could wearing sun glasses alleviate some of the problem?
In VMC (albeit probably IFR) is reducing your limited field of view a good thing to be doing?
If you fly IFR (in VMC) does that absolve you of all lookout responsibilities?
Why not have bone domes, liquid conditioning vests and adequate re-hydration delivered by the CSD?
Try sitting under a green house canopy like the F-15 for 6 hours plus and see how hot it gets, not to mention trying to pee in a bag while strapped to a bang seat.
Sorry, but I think its incredibly bad practice to obscure your field of view.
T