Thanks gents for all the helpful input!
For the record, I've been told that there are no prohibitions against using the river tiles and bridge components in a map intend for use in the MA - just that there are no guarantees that they will work as intended or desired, either.
So I'll keep all that in mind and work around the limitations as necessary when necessary to achieve the desired effect, if possible - although I am leaning towards dropping the longer river I'd intended to place in order to keep the number of exceptions down.
I will definately hold off laying and cleaning up any more of them, though, until I get all the fields placed and can do a full strat build, so I can see how much goopy poo I'd be "stuck" cleaning up and/or working around.
Guy