Swedish newspaper, Aftonbladet, dated 9 January 1979 stated that sometime during their New York City stay (but prior to 9 January), ABBA had been doing “ 'uncommercial television advertising’ for the United Nation’s Year Of The Child”. 

Whether that “uncommercial television advertising” is simply a half-joking way of saying that they’ve been talking about the UNICEF show on news shows etc., or if they’ve actually participated in some kind of commercial, is unknown.

The article also stated that while they were in NYC, “they will maybe have the time for a recording with Donna Summer”.  It's again unclear if by “recording” the reporter does not mean a “recording for release on record”, but rather taping something for TV. 

From Billboard - 3 February 1979: “In addition to performing at the benefit, many of the artists involved helped to promote the show. Some taped segments for the Today Show and Good Morning America, while others made calls to a number of radio stations around the country, explaining what they were doing.”     It doesn’t mention ABBA specifically, but I guess they may have been some of the artists pitching in.

From Svenska Dagbladet - 11 January 1979, wrote that ABBA had made (contributed to, presumably “a commercial” for the UNICEF record. No further details.

If anyone can help make sense of these newspaper tidbits, I'd be grateful for some clarification!

 

Thanks to Carl Magnus Palm