Thanks for the feedback.
Although not exhaustive, I’ve looked at various public nameservers (Google, Control D, Quad9, OpenDNS Home, Cloudflare, AdGuard, CleanBrowsing, Alternate DNS), and they all seem to cache TXT and NULL records if the authoritative server’s answer matches certain criteria.
If the inclusion of a custom DNS lookup is a blocker though, I’ll not pursue the TXT/NULL record approach any further.
Something that just came to mind but I haven’t fully thought through: One could encode the data inside AAAA records (with a reserved prefix perhaps, to avoid confusing them with actual IPv6 addresses). In this case, the lookup in Bitcoin Core would remain unchanged (ie, getaddrinfo); there’d just have to be some extra decoding logic triggered by the reserved prefix.