I'm confused. What do you mean by this? Does dnsmasq not put the names of DHCPv6 clients into its hostname database? If ISC DHCPd is commanded to update DNS, does it only update for DHCP clients and not DHCPv6 clients?
You pointed out one way - justuse DHCPv6, but that looses some of nice SLAAC properties.
A different way is to run mdns and let the devices announce their own hostnames.local.
Different tradeoffs, but in practice not too difficult to get to work.
I guess one could even do both...
If that's the case, then you've got to think of SLAAC as operating exactly like IPv4 address autoconfiguration (sometimes called "IPv4LL")... except that you usually get globally-routable IP addresses out of it.
If you want the management niceties that you often get when using DHCP, then you have to use DHCP.
Some very loud purists might say "SLAAC is the only way to use IPv6!". I completely ignore the convenience of LAN-side prefix delegation and say two things:
1) "Good luck with telling your IPv6 clients about things like your preferred NTP server."
2) "For ages, Router Advertisements have had entirely independent 'autoconfigure your addresses', 'use stateful configuration for your "other" configuration' [0], and 'use stateful configuration for your addresses' bits. It's legal to have any number of them enabled. This is a deliberate choice by the folks defining IPv6."
In general, the folks who scream about how IPv6 NAT and DHCPv6 should not exist and should never be used should be ignored... at least about that topic.
[0] Things like NTP and DNS that other good stuff that DHCP can be used to tell hosts about.