I'm weighing in a bit late on this; I just re-read the thread.  The project is to convert DC metadata to MODS, and the specific case in question is a set of letters that are declared to be related,  with no specific relationship cited.  So it is really irrelevant whether or not it is appropriate to declare these to be related in MODS, they have already been declared to be related via DC and that information should be preserved when converting to MODS.
My suggestion is to omit the type attribute.  It isn't required.  Omitting the type attribute  is the way to say that items are related but the relationship is unspecified.
> really should be listed under relatedItem at all.
> however, is not identical.
> content relations, which may be my problem here.