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pg111 - I think the BookCAT people would have been be best placed to do that, but...
AFAIK developing and implementing custom protocol handlers in Windows is a mid-level development task. If it wasn't, me or someone else would have done it for calibre

Search for "creating a Windows protocol handler" and read all about it.
Curious: what can BookCAT do that you don't think you can do in calibre?
I believe BookCAT uses MS Access, so a plugin might be able to use something like ODBC against its database.
If BookCAT has an accessible API it may be possible to develop a calibre plugin that utilises it. The Zotero and Mendeley plugins might provide a rough guide on the feasibility of this approach. The Zotero software publishes an API which the calibre plugin uses, not sure about the Mendeley plugin, but I think its the same.
Or, if BookCAT has command line interface you might be able to use that in a plugin.
BR