People do need to separate out SQ flights flying in and out of US vs. SQ flights out of other countries for this particular issue. The difference in availability between the two cases is striking. Either because SQ sells almost all of F from the US or because there are a lot of redemptions from the US, it is very difficult to get SQ F with US origination/destination.

Origination from other areas is not very easy but not impossible either with the usual flexibility you will need in any of these type of awards. In fact, there was an SQ flight out of SIN I remember which had no availability in SQ Y but had availability in F.

So an approximation is to try and get a *A award with a connection where you use a different carrier from US. This may be a lot easier from the East Coast than the West Coast, of course.