With the four Halloween tonics available from the event (plastic spider, Halloween, concentrated Halloween and candy corn) there seems to be a simple but elegant solution for those of us that the RNG gods just dislike:
Mystic Forge recipe: 250 of each = one endless.
The sheer amount of bags required to even obtain 250 of each puts a massive limitation on this, the account-bound nature of both the tonics themselves and the endless tonic ensures limited impact on the economy, and it at least would make opening your 50 thousandth bag feel like you’re at least still progressing towards something.
It just seems like four slots in a forge recipe and four different tonics is a match that was meant to be, doesn’t it? 
Originally posted in the Halloween forum which has been archived, probably should have been here from the start anyway!