PeteWild: "Open Source licensing of RISC OS would almost certainly be under a bespoke licence with appropriate terms for use, charges and feedback of sources."
Then it won't be open source. Not even Microsoft labels such licences as open source, and any pretense that such licences are open source will be met with ridicule and, in a slightly different world, a trademark infringement lawsuit: the OSI probably haven't asserted their rights aggressively enough to get misusers of the term (in the manner suggested) into immediate trouble, but such misusers would be courting distaste if not danger at the very least.
"a. Minimal royalty for personal use"
"b. Per unit Royalty structure for commercial use"
"c. Feedback of sources in such a way that it can be re-licensed by Castle (i.e assignment of ownership of Derivative works to Castle)"
Actually, you could get away with asking contributors to license their code in different ways, but since the community is apparently just there for donkey work, you want them to give you their work for whatever future purpose. Suggested licence name: Fanboy Special Helper Source Licence.
"d. Prohibition of use in certain areas, e.g. Only for use on ARM processor hardware, certain excluded/controlled application areas."
To sum up: not anywhere close to open source; even some Microsoft shared source licences are more open than the above.
JGZimmerle: "I don't see point (c) as such a big problem, as long as Castle has to publish any changes they make to the open-source-components as well. IIRC that is what MySQL AB does."
Companies like MySQL probably either pay for contributions that they want to offer commercially (under a proprietary licence) or they rewrite the functionality in-house. Way before the OSI, there were licences in circulation (perhaps more like permissions statements, actually) which requested or demanded modifications to be made available to the author, but this is actually incompatible with even "strong licences" like the GPL - possibly contrary to popular belief. Even if (a), (b) and (d) were dropped and such a clause were introduced in favour of (c), people would still shun the project... and deservedly so.