Code has now been committed to pginstaller to move user creation to the execute sequence. This means it can be used from silent installs, and also that the user won't be created until you have filled in all dialogs - in 8.0 we create it as soon as you leave the service config screen.
If you need this, or really anybody else as well, please grab latest cvs, test and let us know how it works. If you can't compile yourself, wait for the next beta which hopefully won't be too far off. But as this is a fairly major change, we'd appreciate a lot of testing.
A couple of minor fixes have also gone in, as we're nearing release. Not a lot of bug/feature-requests still pending now.