1) Yes it is possible. It is quite a common thing to be done.
2) I would fill it with a block of wood of the same variety as the rest of the bass.
3) If you fill the hole properly, the repair will be invisible under solid color paint.
4) No, it is not complicated, at least as lutherie goes.
Here is a rough rundown of how I would do this.
1) Expand humbucker cavity to be removed into the smallest rectangle possible. This includes routing it slightly deeper to remove any finish present in the bottom of the cavity. (This is to make the filler block as simple as possible, and to get rid of any finish inside the pickup cavity.)
2) Make a rectangle filler block the length and width of the cavity, but slightly deeper by about 1mm.
3) Fit the block into the cavity by sanding and test fitting it until it is snug. Glue (Titebond 1...red bottle) and clamp, leave overnight.
4) Plane, sand, scrape the filler flush to the surface of the bass body.
5) Spread filler epoxy into the gaps, sand flat and flush. Repeat as necessary to produce a flat-to-the-touch fill. For the filler epoxy, I would use System 3 quick cure or mirror coat, or Pacer z-Poxy.
6) Refinish the whole body.
Last edited by HaMMerHeD : 12-23-2013 at 09:46 AM.