Concerning TXT records, I actually tried them first because I thought there might be an advantage to having “human-readable” data in the record. I used a base85 encoding to represent the 256-bit keys/hashes of Onion/Tor address. But using human-readable letters doesn’t make the data human interpretable, so I went with NULL records which are more efficient because they allow for binary data.
Good point about the lack of support for caching in the comment though. I’ll look into to. But given the 60-second TTL used by most DNS seeds, I highly doubt more than a handful of bootstrapping nodes ever used cached DNS data.