> See perhaps OPTION_IA_TA (Temporary Address):

I was unaware of this, so thanks. Sounds like it addresses (pun intended) my concern.

> How does DHCPv6 hold back IPv6-mostly? First, most clients will send out a DHCPv4 request in case IPv4 is the only option, in which case IPv6-mostly can be signalled

It's not the signalling that's the problem--it's the configuration of the CLAT which requires SLAAC, afaiu. This is in fact the subject of the latest IPv6 Buzz podcast episode: https://packetpushers.net/podcasts/ipv6-buzz/ipb197-slaac-an...