I think a little scripting would probably be required to get this info on the REQ level.

You would have to use a GlideRecord to check for RITMS existing under the current REQ, with an additional .addQuery parameter that the Order Guide field contains your OG. If at least one such RITM exists (.hasNext() returns true), we know the REQ comes from that OG and the entire script can then evalueate to true (return true value).

If you could post how you are managing the workflow/approval on REQ level, it will be easier to give you some more specifics.