Nice idea. I guess you could still do something similar by looking for verified accounts tweeting with your campaign hashtag(s). Or accounts that are otherwise 'known' to be a celeb - there must be a directory out there.
No, I mean, with twitter you have ids and you can map those to real people. You often won't have a name other than the id, and the id won't normally be in wikipedia. For example, '@antanddec' is the twitter account for two UK tv presenters, but the account doesn't have their full names spelled out. It is verified though, which is usually a clue that they're someone (relatively) famous.
Also, unlike wikipedia, it is unambiguous - you can easily check that it is the celeb and not someone else with the same name (eg @warrenellis, who isn't the musician)
That's probably a good opportunity to make sure that they're comfortable with being used to promote the petition too. The script / using big names to promote the cause is a pretty smart idea.