Brainstorming:
Depending on the quantities you need, you may be able to ask a company that makes biodegradable plastic containers to manufacture a batch of wax tart containers using the same material, assuming it can withstand the temperatures of hot wax being poured into them.
Forgive my ignorance, but could one make the wax tarts in a separate mold, and then package them in a single cavity clear plastic clamshell so they are still visible? Or does it require the clamshell be mold as well as packaging? Or cellophane is compostable - perhaps a cardboard mold or backing that you can then wrap in cellophane?
If you are unable to find what you are looking for, at very least PVC (looks like what a clamshells are often made of) is recyclable. And if the molds are reusable, you could offer your customers the option to return the molds - like when you get milk in a glass bottle and it has a bottle deposit.
I'm sure someone else with more experience will be able to offer better advice and information. I wish you luck with your endeavor!
