Mmm. Might have spoken too soon
The 32-bit version does seem to be working - and reliably - for me. In 64-bit mode, although I now get non-zero values for lap time, position etc. (using the same segment mapping etc. - I just turn 'force 32-bit mode' on/off in rf2) - the values I get are flaky.
The position indicator sometimes shows P64 (starting at the back of a 15-car grid) and sometimes P15 (multiple restarts of rf2/races). Whichever it decides on, it doesn't update when I pass someone.
The lap time display is similar - it seems to choose a random value (possibly whatever was last showing when I ran it in 32-bit mode) - but not update it.
The speed and revs readouts (revs on right segments - showing leading zeros) are live, accurate and do update just fine.
For what it is worth, I have tested both with and without my other usual 64-bit plug ins (I move them out of the relevant folder rather than just rely on switching them off in-game).
I'll be interested to see what JonRarit finds when he tries it. I'll keep testing to see if I can narrow this down.