Devsfan,
I suggest you stick with the plan you have made, call FinCEN for advice at (800) 949-2732, and document the conversation. My approach would be to provide them with my suggested response rather than just asking the question straight out. Your argument is that cashing the check is simply an ancillary service which is directly related to the wire transfer, the service for which they are already registered; i.e. no wire transfer, no cashed check.
If you get the answer you want, it would be nice if they said they would write you a letter to that effect, but it's not likely. Failing that, get the contact information for the person you spoke with and then write him or her a letter recapping your conversation with 100% accuracy. The purpose of the letter is not to elicit a response, but to provide you with documentation for your stance.
Of late, the folks at FinCEN have been the source of well reasoned responses, but it's not like they have a reference book that no one else has - they are giving out opinions and there is some variation among them. I would not encourage anyone to rely on an undocumented phone call even if he or she was the one who made the call. I certainly would not rely on results from telephone queries reported by others.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.