self addressed stamped envelope. It would be expensive though I am sure, BUT...if you did it in two parts you might get better results.
The first part would be to warn/tell members with their say...summer? issue of the mag that their next issue in 3 months will include a SASE and then try to pound it in their heads that it is important for them to return their updated info including email. At least this way you MIGHT get a much higher return and once you have them...then you have them!....if you know what I mean. The second part is to mail out the envelopes and pray.
I think that if you got that email membership up to date,that it would even help with the elections,as someone in charge of membership could then email everyone a day or two before the election deadline to remind them to vote by such and such a day for the glory/advancement of the club etc.etc..
Look...it sounds like alot of work. It is. BUT...if done properly I think that one could pull it off. Who knows? This is simply an idea.