As an undergrad applying for PhD positions, can I add to my CV that I will be mentioned in an as yet unwritten paper?
It is certainly appropriate to put in a line on your CV about your participation in the project, naming it, and maybe the PI. You can also state that the work is &quot;yet to be published&quot;. Don't overstate the case, of course, but if the paper is not yet written, nor the authorship determined, you can't say, now, any more than that.
But participation in a project is itself valuable for a CV intended for graduate admissions.