Child of a child theme I am building a series of very similar sites, and so opted to run them all from a singe installation of Drupal 7. I have this all set up and working correctly.

As mentioned, these sites will be almost identical, but branded differently - the only changes being different logos, colours etc. Structurally, they will all be a match.

I am using Zen and building child themes for each of these sites, but I would really like to be able to make a 'master' Zen child theme which contains the bulk of the customisations, then each site could have a child of that with the few applicable brand changes required.

1) I could change the name of the parent Zen theme and customise away (and am actually tempted to do that). Its not good form though.

2) I could make one of these sites the 'master' and symlink the important bits to the other sites.

3) I could stop trying to find a solution, and just get on with the job, copy/paste in hand.

Does anyone know if there is a 'proper' way to do something like this? I'm totally flexible on any of the details - don't have to use Zen, for example.

Thanks!
Yes you can do this.

I often use zen as a base theme and have some common things that are always in the sites we're building, so we have a common sub-theme theme of zen, and then each site has a sub-theme of the common sub-theme.
For things like this it is great to use this kind of setup.

So you have:
Zen > Zen sub-theme > Zen sub-sub-theme

In the sub-sub-theme the base theme would be the sub-theme. 

For more detailed information on how it all works, see the Creating a sub-theme page.