not being able to have access to a toilet during the belts on phase
so it's more in your head than in your bowel, isn't it.... but that's not helpful advice, so I would suggest:
Don't focus on the seatbelts-on phase; there might be other moments whit no toilet available. If it's a 'by bus' boarding, and you've to wait 15min. somewhere in the middle of the airfield, getting out and running around on the airfield looking for a loo, will get you arrested...
Just in case you've not yet used them before:
- look closely at your 'diet' at least a day of 2 before
- take an Otilonium Bromide based medicine (like Spasmomen) from at least the night before the flight
- have a Loperamide based (instant' medicine (Imodium Instants) on hand during the 'critical' phases
- relax!!! If you're already now so focused on 'seatbelts on' horror scenarios, the simple fact of the 'seatbelt on sign' lighting up unexpectedly (in-flight turbulence), will be enough to trigger the IBS...
But first & foremost, when in doubt, consult your GP; read the patient info leaflet, etc.