I guess I'm showing my age, but the last time I patched an inner tube we used an adhesive that we set on fire before we put on the patch.
I know that now a days the auto/tire shops use some sort of adhesive that you don't have to set on fire. If the pond liner is the same material as the tire I don't know why it wouldn't work. But, if the formualtion is different and doesn't use the same sort of patching adhesives then you might end up just having to line the inside of the tire with the pond liner and then secure it around the top edges with plywood and deck screws until the "trank" is filled with water. I was thinking about doing the same thing with pre-cast concrete culvert sections or septic tank rings. I was thinking of setting them on a base of gravel with sand over it, like the underlayment for a paver patio. I thought about setting it up so I could run plumbing underneath the pond liner with a drain line to allow the water out by gravity flow into the planter beds, and then use a pump to return it back to the tank.
Good idea on the possible recylcing idea. The only catch is most used tires are going back for recapping or shredding now, the one other source you might look to is the ones from jet aircraft. The ones used on the large cargo and passenger planes are almost as large as the ones from the earth movers, but the DOT is more strict on them being recapped. (Due to the amount of heat and friction they take during landings the DOT will not allow them to be recapped. Blow out problems.)
Well, take care, and let us know how this idea works out.
Kevin
