Its been a while since I've taken strength of materials or studies galvanic corrosion but I do think you're correct regarding a monel rivet. However, if you can the FPUA down by using a backing washer, then one of these high zoot rivets will do the trick. I will have to look into the availability of aluminum backer washers.
I've been told that the builder supplied spars here in Australia now have monel rivets. I've been using monel without problems of breakage or corrosion. They are more expensive than ss, but much stronger and more reliable as they don't work loose due to corrosion. Cheaper than thru-bolting too.
Monel does have some atributes which make it more desirable than SS. Most notably its strength. However, it should also be understood that monel is even higher in nobility (i.e. corrosion resistant - cathodic) than SS. This means that aluminum (which is very low on the same scale, i.e. galvanic) will corrode at a slightly higher rate when in contact with monel than with SS. Bottom line - regardless of what fastener you use, if it is not aluminum, then you need plenty of Lanocote. I have also tried mylar washers between the eye strap and the boom.