I don't know if I'll make the London Open Data Day - but just to mention that we did some work last year for the Financial Transparency Coalition open data working group on possible data standards for CbyCR (alongside BO, and Automatic Exchange of Tax Information). I'm just following up with them to see if the report can be shared yet.
However, as a starting point, the OECD have provided 'templates' for CbyCR in this report (Page 35) which may be a useful reference point.
You could also considering focussing in on extractives industry disclosures, particularly as (a) EITI are moving towards a more data-driven approach to work, but I think need ideas and inputs around approaches to structuring data (their existing data collection templates might provide good input for a civil society database); (b) new flows of data from extractives companies should be coming later this year through the extractives filing service which now has an API for accessing reported data; and (c) NRGI have been doing some work in this area with http://alpha.resourceprojects.org/contribute.html
Looking forward to seeing how things develop more in this area...