I suppose you could always try the old boatbuilder's decking trick in those areas. You put on your last coat of varnish and then sprinkle something like sugar or salt over the surface while it's wet and then let it dry. Once it has dried for a few days, you wash the stuff off the surface, leaving little mini-craters where the particles were. It's hard to say if it will give you as much grip as you want, but it might be worth trying on scrap wood. They also sometimes sprinkle something like ground walnut shells in varnish, leave them in there and varnish over them, but they do show to some extent. A band of carefully squeegeed fiberglass cloth in epoxy would have good texture and be clear, but you may or may not want to introduce that element to your boat. Checkering on the wood could be a very classy option. There are Youtube videos showing how to checker gunstocks, but I've not tried it and don't know how difficult it is.
