Is that it? it feels like it should be more... (oh no, I remember why: "Going Their Separate Ways" is the replacement for Tricky 21 on the Mac and it has two exits)
I'm fairly certain there are no other hidden exits anywhere, incidentally... I do remember specifically checking a few levels for it including Lost something.
As for other ports, here's the list for L3D:
Fun 12 - Alpine Assault Course (2)
Tricky 1 - Jelly Climber (4)
Tricky 2 - Which Exit? (4)
Taxing 3 - Let the Race Begin (4)
Taxing 9 - Which Trampoline? (2)
Mayhem 4 - Ski Jump (2)
Mayhem 5 - The Prisoner (4)
Mayhem 7 - Driving Range (3)
L3D is a bit different from L1 and ONML because frequently there are exits which are inaccessible, so it's unclear whether you can even exit the level using them – this is true of "Alpine Assault Course", "Jelly Climber", "Which Exit?" and "Which Trampoline?". Of the others, "Let the Race Begin", "The Prisoner" and "Driving Range" have three or four separate groups with an exit each, and it's made impossible for one group of lemmings to enter another's exit (somewhat like "No Problemming" – the only one from the original games with an inaccessible exit – "You take the high road" and "Synchronised Lemming"), and only "Ski Jump" actually has a choice between two exits (it's the difference between making a right-turner or a left-turner, though, not very important)... while in L1, all the levels with multiple exits had a straight choice between going to one or the other.
I quite like "Going Their Separate Ways" (not least because All the 6s is a bit disappointing and not very difficult) because it utilises a streaming technique by separating climbers who go to one exit and floaters who go to the other, which isn't a technique you see very often used in Lemmings.