If at all possible, I also vote for more S&R fields.
I have a similar problem as Millhaus except with Polish characters.
I added fonts to my 650, and sometimes I embed the fonts as well and generally it works well.
However, every now and then I'll run into a strange pdf which displays Polish characters fine in pdf but after pdf is processed they get converted incorrectly to something like a' instead of ą, s' instead of ś, etc. Flashing firmware, adding fonts, etc won't help because ePub gets written incorrectly. Transliterating to ASCII is not palatable, because I like my Polish text to have Polish letters where they should be.
I have verified that I can deal with that via S&R, it's just that now I have to do multiple passes.
Fom the UI point of view I think it could be dealt with by having a list of S&R expressions and an "add S&R" button which would add to the list. To make people's life easier for the common S&R tasks like removing -<pgnum>- we could even allow to name and save an S&R for later reuse...