terrible idea since .onion websites can (and many do) load resources from non-.onion urls

Well, for starters, the SIM card in the eCall system is only needed to receive a callback from the 112 service if the call drops, which the system automatically picks up. You can actually dial 112 without a SIM card in your phone, as a GSM device without one can still connect to nearby radio towers in a "limited-service set," better known as "emergency calls only."

I know some carriers have strange quirks in their SIM provisioning systems. For instance, it may take more than a few minutes or require a specific type of coverage (like UMTS) for an otherwise "dormant" SIM card to activate from limited service on short notice.

I found an article about eCall Callback that confirms this is a known problem: https://eena.org/blog/resolving-the-ecall-callback-issue/