The legal route would to be to provide an installer that downloads e iOS frameworks from Apple, so you aren't redistributing anything. However, it would still be quite a bit of work: iOS has a not-insignificant layer of drivers in the not-yet-recreated IOKit kernel framework (although some are open source), and likely many other low-level layers that would need to be recreated.

However, it's certainly possible, but as 'mparlane says, it would be a lot of (difficult) work.

IMO if it's not illegal at the moment, apple will add another clause to their next EULA to make it so if project gains any traction. (Similar to the way they prohibit use of OS X on non-apple hardware.)

It's a time intensive thing and not illegal if it's your own code/implementation. The google/oracle case shows that the api is not copyrightable but the implementation is.