I was doing some research today, and found an explicit reference to receiving Honorarium on a B-1.   This may be of interest to some people coming to the US on a B visa for short visits.  I don’t think this would really help people who are here for a longer-term m-ssion trip.   This is taken from the Foreign Affairs manual of the U.S. Department of State.

9 FAM 41.31 N11.2 Honorarium Payment (CT:VISA-701; 02-15-2005) INA 212(p) provides that a B-1 nonimmigrant may accept an honorarium payment and associated incidental expenses for usual academic activities (which can include lecturing, guest teaching, or performing in an academic sponsored festival) if: (1) The activities last no longer than nine days at any single institution or organization; (2) Payment is offered by an institution or organization described in INA 212(p); (3) The honorarium is for services conducted for the benefit of the institution or entity; and (4) The alien has not accepted such payment or expenses from more than five institutions or organizations over the last six months

I hope this is interesting and helpful.   Remember that this is not legal advice.  It is just a summary of certain aspects of immigration law which may or may not apply to your situation.   I encourage you to consulate an attorney if you think any of this may apply to your situation.