I almost didn't answer since you have so many suggestions already. But since you asked, this is what I would do...
You need to find something to hold very find sanding dust in the crack. If I am going to be finishing it with Target Coatings, I might use the EM1000 sanding sealer, or EM6000 lacquer. I wet my finger in the finish, dip it in sawdust, and rub it in the crack - that seems to get more in, deeper than a ROS, but sometimes I use the sander trick - take off the dust vacuum since you don't want the sanding clean, you want it with accumulated sawdust rubbed into the crack. (An old, almost dead, piece of sandpaper works better than a new one). You don't have to worry about finish on the surface since that is what you will be adding in the next step.
If for whatever reason I don't want to use the finish to hold the sawdust in, I use Titebond 1. It sands off the surface easily, and if I get too much, I can scrub the excess off with warm water. But I like the suggestion for White glue - either school glue (washes out of clothes more easily, doesn't hold as strong) or regular white glue (I know people who use it for woodworking, even making guitars and violins, since it is reversible like hide glue). But for this use, either white glue should be fine. I hate Titebond 2 since it is extremely hard to remove from outside the joint, well enough to not impact your finish. And Titebond 3 is even worse.