I'd like to offer a discount where if you order product A you can buy product B at a special rate.
To use your out-of-the-box example store -- I'd like to allow anyone who buys "A Bug's Life" to be able to buy "Beloved" for $1.00 (without entering a coupon code, if possible).
I poked around the docs and this forum and didn't see this. Many apologies if I missed it in my haste.
With a custom order total module along the lines of ProductDiscount.java or TotalDiscount.java you can add a discount for product B if the customer has bought product A. Rather than changing the price of Beloved, you could offer a 50% discount or a $10 discount.
This type of promotion is a little trickier to set up because at the moment the Admin App allows you to add products to a promotion without distinguishing whether they are of type product A or product B. What you could do is to define product A and B in config variables unless that is, you have 1000s of them.