I have done this with a tent fly that was loosing its waterproofing. I used turps and clear outdoor silicone caulking that resists UV breakdown. Very messy ordeal, tremendous fumes, and too long a process. Luckily I happen to own a full face respirator with organic vapour cartridges. It essentially leaves a very fine film of silicone. Not all materials are compatible with those chemicals. My tent fly is made of PU, which I scrubbed with a green abrasive kitchen sponge, hosed down, and allowed to dry taut under my pergola. I applied a thin layer of my turps and silicone mix onto both sides using a car cleaning yellow sponge, allowed it to dry, taut, in the shade for 24hrs, then reapplied, and repeated until I had 3 coats on both sides.
The tent fly passed the hose test and was watertight for the 2 weeks I used it while working on a remote island on the GBR. Would I recommend it for a trap or other large item...Not really given the strength of the solvents. To give an old pack another chance to serve you well in the rain.... why not. But please work outdoors, use disposable gloves, and give it a few days to air out fully outside the house.