I ran across this years ago in a Class 1,Div.1 area involving an Instrument J box and termination of a 36 pair cable.
The cable was routed thru an EXP Gland into the box and then each individual pair had a slip on heat shrink sleeve installed over them with some sealing goop inside,these were then heated with a heat gun shrinking the sleeve and melting the goop at the same time around the individual pairs ,then this whole mess had a wrap on/lace up heavy heat shrink boot,this inturn also had the goop, if I remember correctly the finished requirement was about 20" long. It was made by Ray Chem I believe but don't quote me on that it was about 30 years ago.
A telephone call to the cable manufacturer brought an ensuing letter into play saying that the interstices of the cable had a gas block inherant to the manufacturing process stopping migration of gases,this when presented to the client relaxed that requirement of sealing the cable end. We only did that one. It must have taken at least 2 hours for that and then the hardwire terms on top of that,plus the box size had to be increased.
It might be worth a phone con with your cable guy/client.
If you are stuck with that requirement,and if I remember corectly it could have been sealed at the non hazardous as well as either option of sealing location the end would have been legal as the outer jacket was impervious to gas migration.
dick