So, after tickling my two and a half brain cells, I think I've found an alternative which will describe both an act of yarn graffiti (so, it's a verb), and, at a push, the thing that is the piece of graffiti (a noun):
Yarnstrike ! ©
And it'll fit nicely on any tags I put on my yarnbombs. Would be good as the title of a yarnbombing book too! (Adds concept to list of projects to do later this year)
Here's the next bit of colour for a Yarnstrike on an unsuspecting bollard for this week. It would be fun to see if it gets moved around from suburb to suburb and where it ends up. Might have to set up a Yarnstrike Facething page so its peripatetic wanderings can be documented by enabling travellers. Now, that'd be something new!
Edit, 29 March: 2011
1. Installed it in the middle of the day yesterday, cnr Rockingham Beach Road/Patterson Road.