The real problem here is that the converter was missing.
This still stands as a question. For now I've just expanded the call, using
vbNullString values where possible; this is a VBScript class (thus the "Me"
keyword), and .Word, .ConfirmConversions, and .OpenFormat are actually
public class properties which have been set to the instance of Word and some
persistent parameter values respectively...
Set Me.Document = Me.Word.Documents.Open(sourcePath, _
Me.ConfirmConversions, True, False, vbNullString, _
vbNullString, True, vbNullString, vbNullString, Me.OpenFormat)