Ha! We just implemented this for our own product. We ended up using Lambdas to be able to achieve this, w/ async rendering of custom Liquid templates for each account.
Congrats mate, that's awesome! Did you create a separate AWS account for that to separate from the rest of your stack? How did you achieve isolation for each script? V8 isolates or dedicated Lambda per script? Any other challanges you ran into? We're really keen to learn from others doing similar things.