Bitmagnet has rate limiting on incoming UDP requests (both overall and per-IP) so I don't know that it would be vulnerable; if there's anything else that should be done to mitigate any risk I'd like to know.
I do not mean bitmagnet here, I mean other bittorrent clients that respond to sample_infohashes.
outgoing rate limits, and also it could exhaust the overall rate with useless traffic.
Incoming and outgoing are both limited, I think the worst such an attack could do is prevent responding to legitimate incoming queries - this shouldn't slow down the DHT crawler in a noticeable way.