AN UPDATE: Unlike the U.K.-based 2-4-1 vouchers, the Terms & Conditions for the U.S.-based 2-4-1 companion voucher continue to state that a Household Account (HHA) member can use his/her voucher for a companion who is not a member of the HHA:
"Companions do not have to be named within a BA Executive Club Household Account. Members can choose to take anyone as a Companion on their redemption voucher booking."
The T & C's can be found at:
http://www.britishairways.com/travel...2/public/en_us
This cannot be done on-line; it requires a phone call to the U.S. call center in Jacksonvill, FL. However, when I attempted to do this today, the CSA received "an error message" on his computer that denied the booking because my companion was not part of my HHA. I pointed out what the T & C's actually state, but the CSA indicated that he was powerless to override the computer. (He said it was a fairly recent change in the rules.)
He obligingly recommended that I simply add my companion to my HHA, and he stayed on the line while I logged in and accomplished that feat. The CSA never questioned whether the companion resided in my household.
This workaround won't work in every situation (for example, if you have already maxxed out the allowed six members of a HHA, or if the companion was already part of somebody else's HHA, or if the companion didn't want to combine his miles with yours, etc.)
The other possible option would be to dissolve the HHA, if you had sufficient Avios balance to pull that off.