I know it's a bit late to raise this, but you may find it relevant.
> Pull-requests don't have to be proprietary. We just need a standard format.
A standard format for pull requests actually exists - that too as part of git itself [1]. Github decided to just redefine the term like they did with 'forking'. Torvalds was pretty annoyed with this [2]:
> Git comes with a nice pull-request generation module, but github
> instead decided to replace it with their own totally inferior version.
> As a result, I consider github useless for these kinds of things.
While git's PR is designed primarily for email, it can be shared on any medium of text communication. Git's PR can also be made from any random git host, as long as the changes are hosted there. This makes it immediately usable for almost every hosted git repo - especially for sourcehut. No registration required.
> I'm not opposed to using email, but right now the UX/UI looks far too lacking.
Again, a problem with current solutions. Most email clients, especially web clients like gmail are terrible for text mails, threaded views, proper quoting, etc - all of which are necessary for a clean email-based workflow. You have to use something like mutt or astroid, and email workflow becomes immediately more enjoyable. While that's what we have now, it doesn't have to end there. Someone could come up with flashy GUI or web interface that's as pleasing as Github's and email workflow will actually become more attractive than Github's.
> Perhaps Sourcehut could also release an email client specifically for viewing pull requests
Sourcehut is actually working on an email client suited for email workflows, called aerc [3].
> but until then I think it will be difficult to adopt
From personal experience, I find adopting email workflow with neomutt or aerc much easier than adopting git itself. Though rebasing is not a strict requirement for email workflow, email patches look janky without it. Learning rebasing is the hard part, not mailing and applying the patches. But the skill to rebase makes your contributions immediately better everywhere - not just for email patches.
[1] https://www.git-scm.com/docs/git-request-pull
[2] https://github.com/torvalds/linux/pull/17#issuecomment-56546...
[3] https://aerc-mail.org/
