I don't speak for the CAA, and hope I never do (that building at Gatwick gives me the willies), but IMHO if a visor is non-permanently attached inside the cabin, there should be no need for a mod application so long as you haven't altered anything on the main structure.
If you drill, cut, permanently stick, or protrude outside the cabin, you'll almost certainly need a mod application. Keep it minor if you can, which last I looked cost £62, major mod costs don't bear thinking about (£104/hr CAA time, with no effective limit, plus the submission has to be from an approved company, of which there are very few).
Peaked caps and bonedomes in my experience don't go well together, but microlights are happy at 500ft, where the worst of the glare is generally avoided.
G