aluminum/brass reaction?

Umm - first question - why?

You're correct that brass will be worse than stainless, but SS barbs are available. See McMaster pp.240 & 245 for examples. A tough plastic would be safest from corrosion - McM has a lot of plastic barbs and compression fittings as well.

Ned Simmons

Stainless and aluminum are also not good together. The zinc anodes won't help much if they're very far from the joint because the local return path for current is much shorter (hence lower resistance) than the path from the Zn anode.

A strong plastic fitting would be best. It would be best to make your own, using a relatively small thru-hole to get plenty of wall thickness. That could be a bushing or adaptor; then the barb (more moment on long barb) could be made of brass.

I would make an aluminum barbed fitting and then hard- anodize it. Anodizing will keep the threads from galling, and presents an electrically non-conductive surface. Probably more screwing around than you want to do.

I have fitted a brass barbed fitting to my outboard's leg. It is higher up though, where the exhaust relief port is located. My outboard is in a semi closed well and this port would fill the well with smoke and starve the motor of air. The barb has a hose fitted which leads the fumes out the transom via another ( plastic) barb. There are no corrosion problems after about 6 years use, mainly because the barb gets well oiled from the two stroke exhaust. Klaus