Can I cook skinless salmon on a

You can.....but I would trim some parchment or foil just a hair larger and leave some wings on each end for easy removal. Oil the paper or foil, season the "downside" lightly, then plank cook as usual (no turning), keeping a keen eye on the internal temp. I slide mine off the plank off at 140*. Here is some brie with the parchment and wings so you can see where I'm coming from.

Well, foil will work and maybe save you a trip.

Hey, you can also use several thin lemon slices underneath too. All you are trying to do is not allow the skinless fish from sticking. (the foil or paper just lets it be removed without breaking.)