IF there are any spare dongs, baht or dimes rattling around in your sock drawer after your last trip abroad, Cathay Pacific might be able to find a better use for them. If you fly on one of the airline's long haul flights between now and October 31 - or if you're near the Cathay PR department in Central, you can donate spare foreign coins or notes to UNICEF (United Nations Children's Fund). Last year $1.4 million was raised for deprived children in the Change for Good programme. ''This year we hope to raise a lot more,'' said Public Relations manager Kwan Chuk-fai. There is a collection jar in the Cathay office on the 7th Floor, West Wing, Swire Building, Central, which is open during office hours.