Quoting from the online help:
Specifies the connection string for the named data source, where name is either maindb (for the main database) or the name of an external lookup. If the "name=" part is omitted then maindb is implied. For example, "/datasource:Lookup2=c: estMyData.mdb" would set "c: estMyData.mdb" as the source for the external lookup named "Lookup2", overriding whatever source was specified in the model.
I haven't tried this myself, and sorry, but I don't have the time right now, so I apologize for the partial solution attempt. Better something than nothing, though, right?
Let us know if this works for you.