Thanks for the comments Andrew (and Wizard). I didn't see them in time for the bonding session, and although I did clean the surfaces well (sanding followed by acetone), I didn't apply the epoxy to the aluminium immediately after its cleaning.

Sure enough, the fix wasn't a long term one. The washers came off today. I'm actually quietly pleased, first because the alignment wasn't perfect, and second because I just acquired a wheel with a cassette on it for conversion of the bike to a geared one for exceptional rides. An extra-long quick release would have been a solution even with the washers glued, but I'm glad they came off, all things considered.

I think this is going to be the best solution. As it stands, with the high quality super-hard White Industries washers, I have to tighten the bolts to such an extent that I fear for the health of the bolt threads in the hub. Some soft steel washers should help here, in addition to being easier on the dropouts (heat-treated 7075 as they may be).