We know the Imperium can manage cloning and limb and organ regrowth including complex organs. I see it as entirely impossible that they could have something that could effectively be taken that would wipe out any rad effects or even something that could cushion the system so the damage is lessened. It is pretty magical, but so is anti-grav and we get that at TL-8 (early stuff in late TL-8).
Or maybe the anti-rad is just something you dose on for the following month and it flushes the damaged cells from the body so cancer can't get rolling. The med tech for regrowing organs seems to me to require fine control and discrimination which is the sort of thing that could lead to a cancer solution or anti-radiation solutions (though in the long run, these might be done with nanites or smaller things.... maybe).
And if the 3I has better shielding materials (which seems likely) and it is cheap, and if grav vehicles can normally climb to orbital heights, then I suspect it *would* be in the shell of the hull and it would also likely include a soft cover with it (which is still effectively open top for vehicle build purposes).
I mean, if I knew people wanted to use grav vehicles to make jaunts to the upper atmo and to the orbit to RV with ships, and I was making air rafts or other grav vehicles, I'd include that. I fully expect Trepidas and Astrins to deploy from orbit but there's nothing other than budget that might preclude that being in an air raft that's open topped.