If you're going to take a step like that, the <current-chain-height>
should be rounded off, perhaps to some number of bits, or you'll allow
DNS caching to be defeated.

Don't the seeds already set small times? I'm not sure we want these responses to be cacheable, otherwise there's a risk of a wall of traffic suddenly showing up at one set of nodes if a large ISP caches a response. (yes yes, I know, SPV node should be remembering addr broadcasts and such).