I have done some more reseach since originally posting this query and
receiving a very helpful reply. Two things have emerged.
Firstly it has been realised that the usefulness of enabling the focus
confirmation goes beyond just using old prime glass, it will enable the
camera to be focused easily when attached to an astronomical telescope. In
low light conditions this is always a problem. Secondly, I am reinventing
the wheel. A product is/was available from Germany, that is attached to
the camera side of an EOS/adaptor and it provides sufficient data to the
camera body to enable focus confirmation.

My partner in crime ( electronics tech and PIC micro programmer ) and I
wish to make a few of these devices as there is a growing number of
canon DSLRs in our astonomical society. We would prefer not to torture a
camera into giving up it's protocol secrets if we don't have to. If
someone else has already done the work and is happy to share the results,
their help would be greatly appreciated.

Regards
Ian Anderson