"Remember Peter Wild's original comments: some of Castle's prospects have been turned off by the fact RISC OS isn't open source. If they use the same definition as everyone else (i.e. more or less the OSD) then open sourcing RISC OS with a licence as described above won't gain Castle many new customers"
This is not guaranteed. You do not know who those prospects were, and you do not know how willing they would be to compromise on the strict (and, frankly, anal) definition of open source. It largely depends on _why_ they were put off by RISC OS not being open sourced - and there coule be many different reasons, any number of which could be satisfied with a not-quite-open-source-but-open-enough-to-satisfy-us arrangement.
What you have to remember is that Castle _do_ know who those prospects were, and therefore probably _do_ know what their reasons were, and it's therefore likely that should they proceed with the move being discussed, they would do so in such a way as to take those reasons into account.