To be even more technical, in Jingo, the Disorganizers experienced a portal fantasy, because they actually did travel between universes. Listening to the Disorganizer from the other universe was what gave Vimes the effect of a flash-sideways without technically having one.
Yes, I was thinking of Granny Weatherwax's experiences. The unicorn experienced a portal fantasy. The elves also did, but for them it's normal.
I think we are at risk of confusing the story-telling flashback, which is merely a section of a novel that is placed out of the usual time sequence, with the true psychological flashback, which is felt in the present but in which the individual feels that they are living through an experience exactly as it happened in the past. Granny's visions are brief moments that are experienced, but since they are the visions of a different Granny, they are flashes sideways.
Granny's flashes are to a Granny in a different but still Discworldian universe. Maladict's flashes are to a potential Maladict in Roundworld. Because they involve Roundworld. Maladict's flashes are similar to Rincewind's brief portal fantasy adventure, which for the purpose of discussion I have treated as a flash but which actually did help him by transporting him to a different relative location, so his fall into the ocean wasn't as catastrophic as it might have been.
(Come to think of it, Rincewind's adventure was not only a portal/flash-sideways, they were also a flash-forward, because at that time the wizards had not yet created Roundworld...)