isign

As long as you have an existing developer certificate or jailbreak the phone you can. You could use something like isign. https://github.com/sauce-archives/isign Or altstore https://altstore.io/ which is literally a locally run alternative App Store.

Hold on… your premise doesn't make sense. Apple's SDK only officially runs on Mac - but that applies to the entire build process, not just codesigning. Anyone who is already producing a Mac build either has access to a Mac build host, which they can also use to sign, or is using third party tools to cross-compile from another OS. In the latter case, there is also a third party tool capable of signing binaries: https://github.com/saucelabs/isign You do need to pay the $99/year for the certificate, but it shouldn't be necessary to buy hardware just to sign things.

https://github.com/saucelabs/isign posted about 5 months ago. It's a reimplementation of Apple's app signing, that works on non-Apple platforms. For a while it felt like nobody cared, but the minute I went on vacation we started getting some good contributions, mostly from the sideloading community. Although I'm not very familiar with that world. Lately facing some challenges with iOS 10. We know roughly what the changes are, but I'm at the point where I guess I have to provide things like a roadmap.

Building and linking is still an issue, but you can do the signing on other platforms now: https://github.com/saucelabs/isign Given that the Swift toolchain is now open-source and being ported to other platforms, I would expect that the building and linking issues will be solved fairly shortly. I wouldn't be surprised if there was something official announced at WWDC.