Do the brand-names care about all the details of (e.g.) RoHS compliance
themselves? I don't think so -- they rather put that burden on their
suppliers, which will supply the actual paperwork alongside the product.
So it would be relatively easy to stipulate in the contract between the
brand-name and its supplier that the latter had to
* list any GPL software used in the product
* provide the source to that to the endcustomer
Optionally the supplier just gives sources to the brand-name (he already
has to per GPL!) and the brand-name takes up the responsibility of
providing it to the endcustomers.