I did come across that snippet that assigns the settable property. Now I see that I should be using input registers instead of holding registers for the points which are read only. I was thinking of input register as "input to the PLC" which would make them writable and holding registers as "output from the PLC".
A checkbox/radio might still desirable in the datasource edit screen for setting the default permissions on a point but using "input register" instead of "holding register" as the register range will sort me out.
Thanks for your assistance,