I do secondary DNS on my DA server for a non-DA master and I do not need hacks to keep it working, but I suggest to keep the configs separated so DA doesn't even see the domains. On Debian there already was an include named "named.conf.local", so I used that. I also keep the zone files separated, just to be sure.
Email service won't be that easy, you have to create a particular path tree and config in /etc/virtual, create a system user for each domain, place a couple files with hashed passwords... not easy to maintain manually.
I suggest in this case you use DA's own features, adding the user+domain as usual, then removing its DNS zone manually. The user will still see the web, ftp, dns etc features on his panel, but only the email ones will have any effect.