Good call on piwik, and in fact for SEO work, you might not want all your sites being linked by GA UA codes ;)
One of the advantages in the past for things like Piwik was the ability to direct inject events into the database. With GA, you can also do this now via their measurement protocol.
Google are providing an alternative to their old ga.js with analytics.js under their new analytics. All non-premium account holders should be seeing this now as a beta option when you setup up a new analytics profile.
The measurement protocol is part of this, you can inject say a download event, a signup or a button click as an event to filter your data. You could even rig it up to an arduino and a pressure pad and fire an event every time someone opens your beer fridge.
It is like the old custom variables, but provides more options with custom dimensions, metrics and you also get to put real descriptions against them.
Even without MP you can use something like
http://code.google.com/p/php-ga/ which allows you to fire pageviews or events direct from code independant of browser activity. So if you had signups coming back via mobile payment processors, fire an event into your site GA that says "mobile registration" and then use this as a goal, BTW events are now part of the goals where up until recently it was only URL based so you dont have to rely on strange named psuedo virtual pageviews.
Get a rebill, then fire that in as well to work out quality of signup traffic sources etc.