Fair enough, but hardly a problem. Open source the non third-party code, and give instructions about how to link to the third-party blobs that you have already

Fair enough, but hardly a problem. Open source the non third-party code, and give instructions about how to link to the third-party blobs that you have already distributed as part of the original firmware. Will it build as-is? Probably not. Will it build if someone really wants it, and is willing to spend some time to get it to work? Likely. Will it have the exact same functionalities as the original firmware? Possibly not. Does it give users a way out of planned obsolescence? Yes.