Mole',
I have a friend that went through the same thing just recently. He went through a few mics of various kinds, not much luck. And from what he found, and what I've run into with other ways of doing this, I would start looking for a noise cancelling mic that would 'fit' in a helmet, then do ear phones in some way. There are intercom sets for motorcycles that may be adaptable. I have no idea about brands etc, for them, you'd probably know more about it than me. And then there's the 'BlueTooth' type thingy. A biggy with those is adjusting mic gain/deviation, which is typically not available with 'BlueTooth' adapters. Is your HT doesn't have a mic-gain then you're out of luck.
The only experience I have with that sort of thing is through aviation which is a totally different animal.
Good luck.
- 'Doc
Do a search for 'Pryme Radio Products' in California. They have a BlueTooth thingy.