I have a SharePoint site that gets deployed mainly through feature receivers.
Part of the deployment, is to create ODCs in a data connection library and
publish preexisting excel workbooks to a report library. The workbooks have a
data connection using the odcs. This all works very well, except at present I
still have a manual step of opening the workbook(s) from the report library,
and editing the connection so that the definition has the appropriate url for
the connection file (to the appropriate data connection library). I would
like to be able to do this last step as part of the deployment feature,
however I can't find an programmatic way to do this.

When you open a workbook, edit the data connection properties and go to the
definition tab - in what class would I find the settings on that page in
order to do this programmatically?

Even if I could set up the workbook to use an environment variable, and I
could create the envrionment variable before deploying would be good enough
if that makes it any easier. When I change it manually I can use an
environment variable like %PROGRAMFILES%, but it expands it and stores the
value in the workbook instead of the variable itself.