Sideloading safariextz extensions that I developed myself?

Over the years I've accumulated a number of Safari extensions that I've developed for myself. These are personal workflow extensions that modify aspects of specific web pages, and add minor functionality (like a small RSS extension that puts RSS feeds back onto a browser toolbar), none of which are appropriate for Extension Gallery submission.


Safari 12 / Technology Preview 58 apparently disabled all .safariextz extensions, including the ones I've built myself, using my very own developer certificate that I paid for. I can forcibly install them again, but they don't persist after quitting the browser. This seems rather ridiculous, and not at all safety-related.


Going forward I suppose I will have to move over to Chrome at last, but for now, is there a way to still sideload .safariextz extensions in Safari 12 and make it persist across browser starts? Especially ones that I've built and signed for myself, with my private key + certificate still in the Keychain?