Theres a similar sealant that works fine with plastics, i believe the storm/cascade blocks were sealed with it. I can't profess to having used it, however.
I only have metal parts in my loop (barr the pump, but it obviously has plastic moulded barbs) so I use this sealant.
edit :: two options -
1) loctite 5331
2) loctite 55 cord sealer
Like thread tape, only easier to use + more reliable.
(and yes, i know someone who works for a loctite distributor, hence the loctite recommendations

)