But if from other evidence, it's either me, or one of my two long-lost brothers on the east coast -- sure, I'd do it -- to let them concentrate on finding who they should be looking for.
Incidentally, the very first use of forensic DNA was when the Brits, investigating a sex murder, asked a whole village of several hundred men for samples, and they all did so -- and no one matched. Until some causal pub talk revelaed that one guy had gotten another guy to donate in his name -- they found that dude, and he matched!