Don't know what configuration you have but can't you 'part-bury' a
regular soil pipe within the stud wall, then build boxing aroud the part
which protrudes (ie 30mm or so?)

If I cannot locate a narrower, or flatter, soil pipe then this will be
my only option.

However, I am sure such a pipe must exist. Maybe I could make one by
stretching a standard size pipe over a heater!

The inside diameter of the WC pan connection is only 72mm so even a
correspondingly dimensioned pipe should not present any technical
problems and would fit within my wall cavity.

I just can't find a manufacturer/supplier though.