I don't see why it should be. It's not specifically blocked by MileagePlus, they just can't/won't show it online anymore. If it's available to be booked by LifeMiles then I don't see why calling in to book with MileagePlus would be any different.

SQ F is usually impossible to book with anything except Kris Flyer miles, but last minute business class is another story. I don't see any availability in the next week, but will have to try to call in and make a dummy booking next time I see it there.

Someone else (or maybe you) tried to call me out once before for saying that SQ C was impossible with anything except Kris Flyer, but I then posted a screenshot in the thread from LifeMiles proving otherwise.