@James Ritter Wait.. you're going to hand-write up to 10,000 letters?! Dude. Are you sure you want to do that? I mean, kudos for the dedication, but that would be too many for me.
But to answer your question, yes, direct mail to folks that are behind on taxes seems like a good strategy to find folks that are in generally poor financial health. I would imagine a reasonable percentage of them (wild guess.. but maybe 25%) may also be in trouble with their mortgages. One thing to consider is whether the folks you're targeting even HAVE mortgages, though. Cross-referencing your original list with that group could mean a higher percentage of hits for your time, not to mention fewer hand cramps.
Out of curiosity, where did you go to get said information? I'd be interested in adapting this strategy for my own marketing model. Thanks!
UPDATE: Ok, maybe you mean behind on their property taxes. That makes much more sense. If that's the case, forget my comment about filtering by folks who have mortgages. If you're interested, this post was linked in the Related Discussions section of this thread: https://www.biggerpockets.com/forums/87/topics/68444-direct-mail-to-tax-delinquent-properties