steven r,
It's a little late in the development game to be proposing this but...
Could you use a 3/8" hose connected to a delta-P switch? The switches are make/break (although they would require de-bouncing with software) and should be compatible with a contact sensor.
I did a quick search and switches are available from Omega:
Omega.com: PCB Mountable Pressure, Vacuum and Differential SwitchesIt would take a bit of tuning of line length and diameter but I'd think that the impulse of a tire compressing the 3/8" hose would be more than enough to trip a 2 PSID switch. You could add a blead at the outboard end to refill the line should any "ambient air" escape during the rollover. The switches are rated for 20X proof pressure (40 psid with no damage) so you have a decent working range.
As an alternative, your could use a HVAC blower stack switch if you have any friends in the business. These are rather large but very sensitive - I can personally attest to an "insignificant" ice blockage in the exhaust stack tripping the delta-P sensor and shutting down my furnace. If you're interested I'll get off my lazy duff and look up my furnace spec's.
[TTA Edit: Added description to LINK; added IMAGE.]