I'm building up a very nice 51-style fretless bass with a highly figured korina body and a bubinga neck/fingerboard (photos to come shortly)

Ive been finishing the body with Tung oil. Warmoth says the neck doesn't need finish, but I assume a bit of protection wouldn't hurt.

A bit of danish oil? lemon oil on the fingerboard alone? Rub on poly? Im stringing with flats or tapes.

Wet sand Danish oil on all of it.

Whatever you do, don't leave it unfinished. Humidity changes want to move it around, and if it gets a lot of play it will start to look dirty. Also, bubinga darkens up rather nicely I think under an oil finish...unfinished it doesn't look nearly as nice IMO.

Wet sand Danish oil on all of it.

do you danish-oil the bubinga fingerboard as well?

(BTW am i right that rickenbacker fingerboards are bubinga?)

I would not for protection from the strings but more for beauty reasons. That is one of the great things about using only 1 wood the ability to wet sand it. If you have several colors of wood they all bleed into each other. We will see what others suggest..tom

(BTW am i right that rickenbacker fingerboards are bubinga?)
Depends on the model, and the point in their history. Some bubinga, some rosewood, some "vermillion" (padauk, I think), maybe others, too.