Client has multiple subsidiaries with different use cases. One only 1:1 relationship between bills and payments and prints them from the print checks screen, not the bill payments. They need bill line detail to show up on the step. The other subsidiary pays multiple bills with a single check and needs the bills in the stub, not the line detail. Line detail from the bill is not exposed as far as I can tell in the Advanced PDF engine. Any ideas? I can easily enough have a script set the default form for the different subsidiaries but the line detail has me stumped.

Not sure I'm fully understanding the issue but sounds like you need to script to add in some data to the transaction that your Adv PDF can then pick up. If they are using the Print button you can have a UE script run just in the PRINT context and add the relevant detail for the purpose of printing.

Yeah they're using the Transactions > Management > Print Forms > Checks page

That sounds about right.