The canonical tag was launched in 2009 and has been used by many developers since then. But I've never seen it mentioned for use for Google AdWords landing pages and controlling Google's AdWords spiders.
A WebmasterWorld thread asks if it can be used to handle multiple landing pages or not.
From what I understand, Google's AdWords AdBot does not support the canonical tag. At least, I cannot find any mention of it in Google's documents, nor can I find any posts from AdWords advertisers claiming they have used it successfully.
The only thing I can find is a post from 2009 with one advertiser claiming it has messed up his Quality Score for his campaign.
Forum discussion at WebmasterWorld.