Here are a couple of processes for larger cubes, plus the
requested edge pair flip without disturbing centres (p2),
as well as a minor correction for the 5x5x5 process:

4x4x4 processes (measured in slice moves)
---------------

p1  Flip LD edge pair   (r3 D3) ^3 + (r1 D1) ^4 + Rr3 D3 R1 D1 r3 D3 R3
(disturbs centres)          D1 R1 D3    (25)
p2  Flip UB edge pair    r2 D2 l3 D1 R3 U1 R3 U3 l3 U1 R1 U3 l1 R1 D1
(retain centre positions ) + U2 r1 (u2 r2 l2) ^2 + r3 U2 r2     (26)


5x5x5 processes (measured in slice moves)
---------------

Flip 2 middlemost edges at FD and BD with:
p1 (fm1 D1) ^3 + fm1 D2 + (fm1 D1) ^3 + fm1   (disturbs some centres)
followed by:
   D1 + (fm2 u2) ^2 + (fm2 l2) ^2 + D3    (corrects the centres)  (25)

-> Mark
Email: mark.longridge@canrem.com