I think the key to making this work is to make sure the connection to the base is absolutely solid, and the base needs to be strong and inflexible. If you're going to be hand planing, you ideally want to have zero racking of the bench during that operation. It's also good in hand planing for the overall weight of the bench to be heavy enough so it doesn't shuffle along the floor or whatever. If the bench is strong enough but not heavy enough, you could potentially attach it to the floor or a wall. In instrument making, chances are you won't be using a lot of wood with big knots, grain reversals, etc. that require a lot planing force, or have to hog off a lot of wood from work pieces, so the requirements for bench strength and mass maybe aren't as great as in general hand tool woodworking.
If it's a solid-core door, you have a decent chance that this will work. A non-solid core door might also work, depending on the location and dimensions of the solid parts in the door. Attaching the base to the hollow honeycomb part probably won't be satisfactory.
It needs to be "flat enough" in the area you plan to use for planing so that it can be a flat reference surface. For ukuleles, the area that needs to be flat enough is not very large - a little bigger than the typical size of front, back, sides, neck, etc. For making furniture with larger parts, it's good to have a bench top that can be planed flat if it doesn't start out flat enough. This isn't going to be possible with a door.
With all that said, I do have a work table with the top made from a light-duty hollow core door, and it has been a useful thing to have. I just don't use it for planing.
EDIT: Just noticed that this is a zombie thread, and the OP has probably long since solved his workbench issues. But perhaps someone else will find the advice useful....
