Hi, Tracer!
I understand your frustration. IMDS originated with the European Union (EU) End-of-Vehicle-Life directive and since the intent was to define all substances entering the EU, ensure recyclate markings, and identify any restricted or reportable substances it may be bleeding over as a requirement to other commoditiies like toys and imported household appliances, etc.
Working as an OEM Tier 1 I have generally helped out sub-tiers by simply checking the part mass, the sub-tier part number, and material composition. I would then go to IMDS and use materials that were already created by one of the standing IMDS organizations (IMS itself, S&E, CAS No. Search, etc.) and make a node for that component.
You may be able to persuade your customer to help you out by performing a similar service.
Best of luck!