They're a little fiddly on a planer but it is possible. You have to have the feed on the rear bed just behind the cutter block to achieve any sort of level of straightness as if you have it in front or on top of the cutter you will just plane a bow into your timber.
I remember seeing someone had made a jig to hold a 4" belt sander to the bed of his spindle moulder so he could use the attached powerfeed with the belt sander for sanding continuous lengths of beading, so you can get creative with it should you need to justify having one.