Hi Linda, unfortunately gagging/choking on mucous is one of the side effects of radiation. Kris too suffered terribly. He was helped heaps when the hospital (in NZ) gave him a "VICKS STEAM INHALER - personal electric inhaler. This consisted of a machine you filled with some water and a mask/hood you leant your face into which trapped the steam for you to then inhale/breathe. This adds water to the mucous which loosens it enough so that you can spit/cough it out, This alone helped Kris during his radiotherapy and he used it often through the day/night.
There is a website on the box so I'll put it here
www.kaz.com/vicks
Hope this helps. I'm sure you can source one in Oz.
Tammy

Caregiver/advocate to Husband Kris age 59@ diagnosis
DX Dec '10 SCC BOT T4aN2bM0 HPV+ve.Cisplatin x3 35 IMRT.
PET 6/11 clear.
R) level 2-4 neck dissection 8/1/11 to remove residual node - necrotic with NED
Feb '12 Ca back.. 3/8/12 total glossectomy/laryngectomy/bilat neck dissection/partial pharyngectomy etc. clear margins. All nodes negative for disease. PEG in.
March 2017 - 5 years disease free. Woohoo!