What I generally do (I'm still relatively new to PE).....
For seatbelts and the like I will usually carefully cut them from the PE fret and hold them with sets of tweezers I have that work like clamps (i.e. you have to squeeze them to release the part). I very carefully hold them by the buckle, being careful not to bend it. Airbrush them like normal (many people like to primer first), then when that's dried and cured I hand paint the buckles.
For really tiny stuff like levers and the like I will cut a mask out of some cardboard so that the parts I don't want sprayed are covered, and the parts I want to spray are exposed. Airbrush as normal. If you hand brush, no worries, I'd leave the parts on the sprue and hand paint them. You can then touch up the spot where you removed it from the sprue later with a light dry brushing.
If the levers etc. are the same color as the main cockpit components, I'd attach them first, and spray them along with all the rest of the stuff, then hand paint the handles.