The ultimate result was that calling them at 1-800-4MYXBOX (1-800-469-9269) was the proper request channel. Details of failed contacts follow:
Contact 1: General Windows Live ID/Passport support contact us form:
Results: One auto-reply with no helpful information. Included an invitation to reply for further info/help. My reply resulted in a delivery notification: failed. Nice going, Microsoft.
Contact 2: XBox Live support contact us form:
Results: One (probably) form letter that was reasonable and symapthetic, with the directive that I should phone their support, as they could not do billing related operations via e-mail.
Contact 3: XBox Live support telephone (1-800-4MYXBOX/1-800-469-9269):
Results: Perfect. “Oliver” was very helpful and very professional. I even have an issue number in case I get billed in July.