> One plausible fix for _that_ is to have a <plaintext> tag right after your <script>.

That seems more plausible since `<plaintext>` can't be closed. In fact mutation observers can be used to get and process the partially downloaded JSON before onload (I'm pretty sure this is possible but haven't tested, YMMV). That would be still horrible as a general solution, but might be actually an interesting solution for more limited situations.