Mercury - like inertial ball/ramp switches are no use - As you corner the mercury is forced away from the contacts you need it to short out (close). Centripetal (sometimes mistakenly called centrifugal) force. Makes it very hit and miss - I've tried. I already use an inertial switch for something else so I have some feeling for what affects things.
I'd use twin axis gyroscopic or magnetic field tilt sensors.
going up/downhill during banking might affect the trigger values. This type of sensor is used in dead reckoning GPS devices to control SatNav's when satellites are not receivable. I've toyed with dead reckoning too - it has many other advantages - but u-Blox kits are a bit beyond my needs.
I've mailed Hummingbird in OZ at above address - I already use their GPS sensors. See what they can offer.
The idea is to use fog-type lights or Low Beams mounted at approx 25º from horizontal and triggered at leans of above 15-16º. This should give a nice flat no dazzle beam. THE MATHS JUST HELPS SET THE ANGLES.