A compromise has been reached. Although my advisor prefers to err on the side of inclusion, adv accepted that there isn't enough time in this case (Proceedings submission due next Friday) to get them the prospective additional authors up to speed and give them some input in the submission itself. It's all in the phrasing...inclusion doesn't sound so great once you realize you're telling colleagues "we're putting your name on this, but not giving you the chance to make suggestions or even approve of it".
Should we attempt to get published in an official journal (Proceedings aren't "archival", so rules against double-publishing don't apply), an offer of authorship will be made to those who may feel some shared ownership of the project, with a careful and courteous explanation for why they were omitted from the poster. Which is fine with me; I don't mind sharing authorship, I just feel queasy with the potential of being author on a work you've never seen.