I have been on a number of Jumpseat rides, including 2 since 9/11, both with British Airways/CityExpress, neither with any prior arrangement.
One BHX-ARN on a CRJ and one BHX-GLA on a BAe 146 or whatever it's called....
Pre-9/11 I've jumpseated :
BA 744 in the seat from pre-start until about 45 mins after take-off @ LGW
BA 734 from TOD to gate @ JER
Numerous CRJ into BHX