Sorry, you are right. I didn't know that SQ had been prevailed on to do some, but it looks like it's only been very recent: The oldest photo I can find is July 2004.

Those UA aircraft were from the first incarnation of the Star Alliance scheme. The aircraft was in sections; the operating carrier's livery was at the front, followed by the remainder of the alliance members. When there were too many members for that, they moved to the "washing line" scheme.

SQ's attitude is consistent with their slightly equivocal approach to the Star Alliance. I wouldn't be surprised to see them leave at some stage.