I like the bag idea a lot. I have seen petroleum oils used, but that as well as mineral spirits would make oily residue floaties in the pond(Assuming these irrigation pipe structures are going in the pond). Vegetable oil (spray Pam or regular vegi oil and a rag) would be a much better alternative to petroleum products, but trash bags have good promise. There would be some ripping of the plastic and some plastic pieces will be held by the concrete, but not too big of a deal, it could be burned off if need be. Vegi oil will still yield floaty films, but at least it is organic and will break down in weeks instead of years. Vegi oil will plasticize within a few days to weeks if left in the sun also reducing any oily film when submerged.
You might try one structure without anything as plastic buckets are pretty resistant to anything sticking to them. Let it set up for a few days so long as you don't end up with Customs scenario and try to pull it out, tapping on the bucket with a dead blow. You could even store it in a cool place then let the sun (or propane torch or hot water) heat the bucket up to get it to swell up a bit and it might release even better. Having a taper to the bucket will help a lot also. No taper will likely allow the concrete to swell and lock itself in the bucket.
