Sure. You can't have two plot targets, but there's only one king, so you don't have to mark him, just put him into the plot as ROOT's king-tier liege and only make it available if the culture conditions are met. It has a few drawbacks, but nothing that can't be worked around except for the fact that you can't pick the claimant for a plot backer. There are no hardcoded penalties for plotting against someone, so you can easily make it beneficial to your relationship with the claimant.
Edit: Hold on, I just realized that you can't mod the motivations of plot backers. And so far it _seems_ like they have to have a positive opinion of you and a negative opinion of the plot target. Ignore the above, you'd have to do it the other way around or you'd be getting people who hate the claimant into the plot.