sm_jamieson wrote:
I want an acrylic bath with the overflow hole in a non-standard
location. Since it appears I cannot purchase a bath without a
pre-drilled overflow (at least not without a bespoke pricey item),

Have you contacted manufacturers, such as Ideal Standard, or Twyford's,
to see what they say, rather than a retailer? This might be worth a try.


what
is the method to fill in an existing hole, and finish the surface so
that it is undetectable ?

Hm. I should thing the old "if you can't hide it, make it into a
feature" adage will apply here.