Thanks for your response, back before Christmas. Since then I have been pondering your request for ideas and having a look at what other systems do. The most interesting is, of course, the other major genealogy wiki, WeRelate. They have followed a route much closer to other family tree programs by storing sources and citations in a database. This seems to have considerable advandages (lack of repitition, greater consistency, easier/faster data entry, ability to export this vital data in a GEDCOM file).
After my research, the question I am left with is, why does WikiTree keep this data as free format text? What are the advantages over a database for souce and citation data? I expect there are good answers to these questions, so I am keen to hear your reply.