Hi Larry,
J-P Beltoise was a very good racer in his own rights on 125's & 50 cc in the early 60's but he passengered once on the old Clypse course,might have been a Swiss driver if memory serves me right.I'll look up the records some time just for curiosity.Siffert was also a good rider on Nortons,he also passengered.I was always under the impression that Paddy Driver started once in the S.A.GP, But he seems to have only practised. Takahashi of course went on to have a very good career in cars and still drives in the Le Mans 24 hour race,Serafini was another who had a fantastic career on 2 & 4 wheels but never did the TT although he won the 1939 Ulster G.P on the blown Gilera, Piero Taruffi was another one to combine both.Wal Handley, Freddie Dixon, Norman Black,Jimmy Shaw all raced the TT & drove racing cars in the Isle of Man & elswere with considerable success.Maybe you can add to the list Larry.