It boils down to this:
Only one person can own a file/directory/whatever.
You can give a user permission for one-click installs, and some other control panel features for just that domain. But sharing ownership of a domain is tricky.
Maybe someone else has tried this, or can offer some insight to see if this would actually work:
1) Create a new group, and make you and your user members.
2) Run a periodic cronjob to recursively chmod g+w a domain.
You said that ideally you want to you want to control the domain with your own account. Toss out the ideal and you can control the domain with their account with very little difficulty.