i know you asked this months ago, just got omsi and thought i'd try to answer it,
have you got the saitek profiller software installed? as in that you can assign zones to the sliders, can be a simple 2 zone, and each time the lever goes through a zone, it gives a key press, make the key the one for the handbrake, and your set,
i believe there's a handbrake on and handbrake off assignment in omsi, so even better assign up on the lever to handbrake off, down to handbrake on,
<warning, i ramble on about stuff i'm doing for my sim computer from here on>
i have the same quadrant, bought mine for a fiver on ebay and didnt get any software with it, hence i never knew about the profiler software that allows you to do a lot of good things, i bought my quadrant for railworks to control the levers on the german trains,
then i found the profiler software and used it for the levers on the trains on world of subways 2, setting 7 or 8 zones with keypresses issued according to if i was going up or down the slider range, as wos only accepts key inputs or a raildriver,
im in the middle of making a driving desk for the train sims, making controls that look like the real things (made a horizontal brake handle and reverser lever switch so far from scratch, and modified an old industrial electricity isolator lever switch for a horizontal throttle, those levers are for driving early diesel type trains, think the old class 101 DMU's that you could see the driver working the levers thru the glass partition)
the levers turn potentiomiters, which feed to one of leo bodnars excelent 8 axis, 32 button usb boards, it's seen as an 8 axix game controller so can work with any game that can take analoug inputs.
Anyway, one of my controllers is a button box with a few industrial push buttons, a key switch and emergancy stop buttons, i made it for the subway game, but i now use it on omsi,
perfect for the door controls, bus stop brake, key switch turn the electricity on and off, couple of other buttons start and stop the engine and so on,
i also mounted my wheel 90 degrees up by screwing a block of wood to the table, so it clamps onto the wood like it was the table edge, but the wheel points upwards just like a bus wheel,
the wheel was back to front of course, so i unscrewed the button pad, 4 screws under that and whe wheel comes off the shaft,
unfortunately it's keyed with odd spaced screw studs, but i managed to make it go on rotated 180 degrees by threading the screws in and tightening one by one till it pulled in onto the shaft, will prolly never come off again, but i dont play race sims, truck and bus sims are for me.
it's an old logitech driving force wheel, only 180 degrees of turn, my plan is to get a GT wheel when i can afford it, then later on i'll get a steering colum and wheel from a scrapped mini, their origional 1980's steering wheels were like bus wheels,
then i can get a force feedback steering shaft and motor from an arcade machine, attatch the shaft to the steering colums shaft, which will be bolted to the table,
then a servo amplifier, and take the circuit board out of the GT wheel, motor output goes thru the servo amp, which drives the 90 volt motor for some serious force feedback, (they can apparantly break thrumb if you set the feedback too high!!), the rotary encoder from the GT wheel goes on the end of the wheel shaft, job done, realistic bus and truck sim wheel.
then i'd wire up the indicator and wiper stalk switches into the computer, and would just want outputs to illuminate some of the buttons to make it better,
then i'd use FreeTrack (google it
to control the trackir part of the game to give head tracking, (uses a slightly modified webcam and some infrared led's on a cap to do the same job as a trackir setup does, but instead of costing £150, it costs 36p per led, about a quid for a battery holder with switch, pennies for resistors, a few minutes time taking the IR filter out of an old webcam, and a little soldering and gluing the led's to the cap... would have to be a genuine bus drivers hat of course
no burberry baseball caps here, were not thrashing a vaxhaul corsa around in GTA
anyway, i've rambled enough, hopefully answered the origional Q, and confused the heck out of everyone else, but maybe given some a few ideas, as i've seen at least one person on here is making a bus cab sim for this game.