My feeling is that projects like this need momentum. The more people seen to be working on bits of the OS, the more attractive it is to others looking to join in; hopefully building this up is just a matter of time.
I share your desire for development of ARM hardware, and I hope given - as stevek points out - that the point of opening RISC OS was largely about attracting STB licences, that the project will also help drive native hardware too. I wonder whether the 'promise' of an eventually totally open platform is enough to attract STB makers in itself?
One of the great things about ROOL is that the benefits will never go away; the code will always be available to look at and improve.