And there is precedence out there which covers derivative work like what is suggested. In particular, in the original days of coding PC BIOS, anyone who understood the workings of a PC BIOS but had never ever seen, touched, or slept with someone who had seen or touched any disassembled PC BIOS code was preferred. Doings so meant that the work done on the clone BIOS would not be tainted by the IP of the original.
This abstract also illustrates why Windows NT could be considered a derivative work of Vax/VMS, since the guy who designed the NT Kernel also wrote VMS, though he went in a completely different direction with NT than he was allowed with VMS.
OS4 devs have seen the original Amiga source code, which is AI's IP. Were any of them to jump on-board with MOS, they would bring with them the experience and knowledge of AI's IP, and the temptation to incorporate that into MOS would be too great to ignore in maybe the dev sense and definitely the legal sense.
This is the exact basis of the SCO lawsuit claiming Linux is a derivative work from System V Unix. Not so much that Linus Torvalds had used System V code, but that developers had contributed source based upon their knowledge of the System V core. SCO lost for other reasons (mainly that they don't own the rights to System V, anyway, as it still remains in the hands of Novell.)
Which makes me wonder how the licensing for OpenSolaris works since Solaris is a direct descendant of System V. (Or is it System IV?)
For a project as proposed be able to work, AI would have to release the current OS4 developers from liability for developing an a-like work of AmigaOS, agreeing that the devs can only take with them the knowledge of the deep inner workings of AmigaOS, but not any copies of the original source or internal documentation. Which we know simply will not happen.
It proves to me that AI has no intention of doing anything useful with AmigaOS. Hyperion could push OS4 into new territories which would not only please its current fan- and user-base, but also possibly open a new market for embedded devices, with AI's blessings. Instead, AI binds their hands and limits them to only developing for sanctioned hardware which is either way out of date, unreasonably expensive, or non-existent.
Were AI to take OS4 away from Hyperion, AmigaOS would be consigned to atrophy under the tight-fist of AI (worse than it is now) and ultimately die a permanent and painful death. Well, until AI sinks and someone else purchases the IP.