Is it possible / how to install the bacula windows client on remote Windows XP or 7 machines?
They are all within a domain. Would I need a custom installer, or is there a way to push out the standard installer?
You will need a custom installer. Build the .msi package yourself. Then you can then distribute it trough GPO.
I have done this very recently to push an update out for a few thousands clients. Yes, you need a custom installer to do this. The 'silent' install is not quite so silent and is prone to erroring, particularly where an existing deployment already is available.
Another big problem you'll have is silent installs will generate a 32 byte random string as the password, the standard installer offers no easy way to avoid this.
To solve this I created a silent installer which took arguments to set a specific director and password. I also squashed some of the bugs pertaining to existing deployments being upgraded.
All in all - without altering the windows installer to do this it will be pretty difficult to do this without invoking some other scripting language to alter text files.