Mike,

I am installing a whirlpool tub into an alcove, where the center wall
is an exterior wall. There is room to have about a 6" surround on each
side of the tub, or I could have it off-center and go up to 9" on one
side. SWMBO wants the fixtures on the middle (exterior) wall

Are you talking about mounting the fixture on the back side of the tub?
If so, that would be very awkward to use unless you have a small tub or
plan on climbing in the tub just to start filling it with water.

Is this a tub in a platform situation where the fixture mounts on the
deck, or it is a regular tub where the fixtures mount on the wall?

You mention it's an alcove. What's on the two end walls? Could you put an
access panel on the back side of one of the end walls? Our guest bath has
a closet on the other side of the fixture wall, which made it easy to
hide an access panel inside the closet.

The other alternative is to just install the fixtures on the front
side, but you'd have to get in around them, and they wouldn't be as
nice to look at.

I mounted our master tub faucet in the front left corner of our tub
platform. It's easy to access, out of the way when climbing in and out of
the tub, and doesn't get in the way if we both take a bath together (a
person at each end).

if it's even possible with a topside plumbing access.

I wouldn't recommend top access if you can avoid it. It's just a
potential place for water to seep into.

One end of our master tub butts up against the vanity cabinet. I
originally planned on putting the access inside the cabinet, since the
front of our tub platform had to be waterproofed (we have a large walk-in
shower outside the tub). In the end, I installed the fixture before I set
the tub in. Any typical repairs needed to the faucet can be accomplished
from above the deck anyway. And if I need to repair the plumbing for some
reason, I figured I could remove the tub, or pop a hole up from our
crawlspace. Either way, that's no different than repairing the plumbing
anywhere else in the house.

Anthony