"Is there any way to (semi-)automate this build when we do a new
groupware release?"

Yes, it's possible. I run a script which re-packages horde pear 
packages to rpm as they pop up in the channel. I can add a build 
target "image" to obs which builds essentially the same VM  as the 
SUSE Studio. The pro is the automation. No need to manually trigger 
the build. The con is there is no nice GUI - I'd have to provide and 
update a download link on my own. But since the obs has an API, it may 
be scriptable.

Probably outdated due to the current suse build service stuff.

Seems to work fine so far. I suggest that you switch over to install 
the Groupware, now that is has been released. It might be easiest to 
run the install script with expect.
Of course the missing IMAP server has to be installed too at some 
point. Big bonus points for SQL integration so that new users can be 
added through the Horde user management.
Is there any way to (semi-)automate this build when we do a new 
groupware release?

Your wish is my command. I have built today's releases into a demo 
appliance for EC2, xen, vmdk & other formats

See http://susegallery.com/a/BZ4vzQ/kde4-horde4-demo-setup

Create a .ami for Horde so that people can try Horde out on EC2 with 
no additional setup.