Just a thought - get a big-*ss tarp
, and put it over the trailer before you load the boat the last time this season. Pull it up around the boat, and you won't have to worry about overspray.
Alternatively, leave the boat in the water for a couple of weeks before you pull it out the last time. Leave all the marine growth on the hull - once again, you won't need to worry about overspray because it'll all come off when you scrub the gunk off.
Other than that, I don't have any great suggestions. I almost did mine this summer; was just going to rattle-can it.