Standard unix-style input accepts ctrl-^ as ASCII RS and ctrl-_ as ASCII US. If you want your terminal to accept an ASCII US literally — so that you can use it as the -t argument to sort, for example — you would use ctrl-v ctrl-_ to give it the literal character.

  $ hd
  ^^^_
  00000000  1e 1f 0a    
  00000003

  $ sort -t "^_" -k 2,2n
  a^_42
  z^_5
  n^_7
  z5
  n7
  a42

I guess it depends on a text editor. Mapping a key to insert that character is one possible solution.