If several standby cabin crew members called in sick and there are no more available can standby pilot called to perform cabin crew member duties? Probably it would be more economical than postpone flight let say with 200 passengers...
For the U.S., can a non-cabin crew act as a cabin crew? Yes, except for safety duties.
U.S. air carriers periodically use company employees in the cabins of its aircraft for the purpose of conducting certain passenger service activities, such as serving beverages, conducting customer relations, or acting as translators. These persons are not assigned to flights to perform safety duties. These company employees are not acting in the capacity of an F/A nor are they, in general, trained or qualified to act as a F/A. The regulations do not prohibit the use of non-F/A personnel by an air carrier. However, their presence could conceivably interfere with the F/As if they were not properly instructed.
My interpretation is technically yes, especially that pilots are trained in safety duties.
But that's not to say there wouldn't be a union rule against it.
I'm not sure if this is possible. Cabin crew are trained for a number of things including handling of emergency situations, which are different from what the flight crew receives. Also, there is no reason to believe that the flight crew is proficient in various duties of the cabin crew (like performing safety demonstration, etc) as their duties are distinct.
On the regulation side, the certification requirements are different. For example, the FAA requires the cabin crew to have a certificate of demonstrated proficiency.
In the fall of 2003, Congress established a flight attendant certification requirement in the Vision 100-Century of Aviation Reauthorization Act.
The act requires that after December 11, 2004, no person may serve as a flight attendant aboard an aircraft of an air carrier unless that person holds a certificate of Demonstrated Proficiency (certificate) issued by the FAA.
This would mean that the flight crew can't operate as cabin crew (unless they have the certificate), atleast in US.
The Answer for your question is NO.
Pilots are not trained to serve passengers, but I think that was not What you were referring.
Pilots can and must help flight attendants in emergency, this does not mean they are working as a flight attendant, in case of an emergency the pilots will finish all the emergency check lists turn off engines, APU and Battery as part of the check list procedure and after that will communicate the evacuation of the aircraft. After this command the pilots have to leave the aircraft, so in this case the pilots can help the flight attendants with the passenger evacuation. Sometimes asking the flight attendant to jump off in the scape slide and help the passengers at the other side of the slides. Remember that this is a quick procedure as a time to evacuate any commercial aircraft is 90 seconds by regulation. Hope this help ;)