I think Netsurf's model proves that cooperative development on a project from scratch /can/ work on RISC OS when the conditions are right to do so. The result in Netsurf is IMHO extremely impressive. Most RISC OS developments are from a single author; this is quite the contrary from other platforms where collaborative development is the norm.
I think the question is more how you foster the conditions for cooperative development to work. We should learn from Netsurf's success.