has anyone tested if arbitrary third party extensions (non mac store ones) are installable? and if not, how are developers expected to write & test mobile safar

has anyone tested if arbitrary third party extensions (non mac store ones) are installable? and if not, how are developers expected to write & test mobile safari extensions? i'm pretty skeptic of the prospects that apple would open up a hole in their walled garden on their own initiative. it's not like they're (currently) under any pressure or competition there.