"two PIC compilers ... allow the placement of structures on ports, but both have strayed from the ANSI specs to achieve it (presumably)"

No - that's not what Jon et al were saying.
The point is that the ANSI spec gives the implementation complete freedom to allocate the bits in bit fields howsoever it sees fit; in particular, the allocation could change between compiler releases, or with different compile options, or between compilations, or even between variables in the same module!
So, unless your compiler manual specifically guarantees a particular organisation, it is unsafe to rely upon it!