I was looking at replacing my bathtub with a slightly wider, deeper
unit (32" wide over 30") - but space is so limited as is in the
bathroom that instead of taking up more floor space I was wondering
about going the other way and altering the wall behind the tub.
As I'd only need 2 inches more space and don't want to intrude into the
adjoining room, I was wondering if I could rebuild the wall other than
conventional stud construction

Assuming the wall isn't load bearing, it sounds doable, just a lot more
work.

You could reframe the wall with studs turned on their sides, but then
you'll have to sheetrock, paint, etc. the room on the other side. If I were
going this route, I would find the straightest KILN DRIED 2x6 studs I could
find. These would offer slightly more strength than a 2x4 layed flat, and I
would think it would offer better resistance to twist if you anchored each
side well.

If you want to try avoiding damage to the wall on the other side, how about
cutting the original studs back 2"? You could set your circular saw to a
2" depth, then cut notches every 1" or so. Then knock out the pieces. If it
were me, I'd add a layer of 3/4" plywood, SCREWED every 6" to the studs to
add extra strength. Then install your tub. If you're tiling, put hardiboard
or durock over the plywood.

Anthony