Why doesn't everyone who would have subscribed put their cash into a fund to buy RISC OS from Castle? Then they could make it available under the GPL, potentially realising any value the software has - indeed, potentially generating value as people start to do interesting things with it.
Meanwhile, RISC OS Ltd. could continue with their proprietary branch of the operating system, just as the various prior agreements permit. Castle could also have some kind of special licensing arrangement.
But I suppose with the major players still living in the world of 1980s microcomputing, we can't expect something as sensible as the above to ever materialise until they're all in liquidation.