Well, I've tried enclosing the bcc recip list in single quotes, and also have tried just opening a pipe to sendmail as suggested, and also combining them, but they all result in an X-Final-To header.

Conclusion: I blame it on the host for running Sendmail 8.9.3 which is (as of this writing) over 5yrs old (and getting that upgraded will be like pulling teeth). Argh. The host shall remain nameless.

Thanks to you who've reviewed this question and offered suggestions.