IMHO, I think you may be trying to find a solution for a problem that could be handled a simpler way. Is your bokken fairly new, to you? If so, have you yet sanded off the varnish coat (with fine grit sand paper) and oiled it with lemon oil or linseed oil? I find that an oiled finish is much less slick than a varnished finish, and actually feels good in your hands. Sweat dosn't seem to affect my grip on oiled wood as much it does on varnished wood.
If you tape the handle, you are probably going to develop some compensatory training problems
-- for example, you'll become dependent on the friction of your grip-tape, rather than on your hands and how you manipulate the bokken. Aiki-Weapon traditionalists wouldn't even consider taping a bokken handle (some of them won't even tolerate tsuba...).
I would consider using a couple of cheap rubber hand-strengthening balls, properly conditioning my bokken, and observing more experienced practitioners to see how they "handle" their bokken
, before I'd try taping mine.