While I'll use the term filler for convenience sake, I'm not found of the term. I play shorter games when that's what I have time for, not just to fill time. And I think shorter games should not get held to lower standards just because they're shorter.
I have looked at role-playing games that are designed to be played in a very short time, like game poems that are meant to be played in only fifteen minutes. However, they are designed to provoke very strong emotional responses. To my mind, that is the opposite of what a filler means.
More than that, if we accept the idea of an RPG filler, where do we draw the line? Some folks I know prefer multi-year campaigns. For them, a one-shot might be a filler. However, that seems extreme. What is the time limit? Two hours? An hour? And what about weight? If a game is emotionally heavy or distressing, is it still a filler?
When I actually think of the word filler for a game and mean it, I mean something like a game I'd pull out while waiting at a restaurant. The Looney Pyramid game Treehouse is a good example of just such a game, with the added bonus that is waterproof.
For me, even a one-shot or short form RPG is something you plan out ahead of time. Someone suggested the Parsely system as an RPG filler. And I can see why. Heck, it is a game I've thought of having at the ready if the GM is late. But for me, it is really a party game, and experience to be judged on its own merits.
RPGs already use the term short form. Filler just seem unnecessary on several levels. Since apparently Tau's use of it didn't catch on, I am not alone in thinking that.
What I do think is a concept that I have seen and is worth exploring is pick-up-and-play RPGs, games that don't require preparation. It's an idea that's been around since the 80s with games like Sandman and Ghostbusters. I've seen designs for rules light one-shots and ones for campaign play. But I can't say I think of any of them as fillers.