This Santo film was only recently dubbed, so there’s probably some very interesting bad movie in a foreign language with no dub, so maybe one could riff over a subtitled film?
Or maybe a silent film? I guess there’d be a lot of “dead air” between intertitles. Maybe just a short?
In Japan, in the days of yore, silent films enjoyed a process called benshi. It is such of which you have described.
Bridget and Mary Jo riffed a short Christmas segment with no dialogue at all. It was completely silent (although I do wonder if there was supposed to be narration at some point). It was hilarious, but I don’t know if they could do an entire movie like that.
RiffTrax also did A Trip to the Moon.
I really like this question — if you’d like, you can resubmit it once the Robot Wars category goes up tomorrow, or link to this post for tomorrow’s Q&A. I would love to hear what Joel and the others think about this, as it’s a great idea.
There’s honestly a ton of potential in riffing silent cinema, even covering films whose dialog is in a language other than English. The only drawbacks I can think of are more stress on the writers’ end and/or a little less accessible viewing experience.
On the other hand, selecting a silent film might make the riffing stand out even more… 
I guess one thing they could do is remove the title cards completely and just create a new movie with their own interpretation, but that’s kind of been done before hasn’t it?
Giorgio Moroder sorta did it with his 1980’s re-release of Metropolis. He turned some title cards into subtitles and let music advance the narrative. I’m sure there’s been many silent films that used overdubbed dialog to create a different viewing experience. Riffing might be a bit different, but it’ll likely have that same vibe.
There’s been a few sound films with all-new dialogue/audio, like SCTV’s episode-long parody of “The Cisco Kid” (that somehow wasn’t done by the actual SCTV cast…that was an interesting episode).
I’m also reminded of Proctor and Bergman’s J-Men Forever.
I encourage anyone genuinely interested in this question to look up Fractured Flickers.
It was an early 60s TV show from Jay Ward Productions, the geniuses behind Rocky & Bullwinkle and George of the Jungle.
The host, Hans Conreid, would introduce short segments, referred to as “flickers”, which would repurpose old B/W silent movie footage with careful editing and dubbing to inject the Jay Ward brand of humor. That was the initial goal, anyway. As production time got the best of them, the quality of the dubbing and so forth slid more and more. It ultimately ended with 26 episodes produced.
Incidentally, I highly recommend Keith Scott’s book, The Moose That Roared, which is about the history of Jay Ward Productions. One of the best books ever about TV production.
I looked this up, and it’s because it wasn’t originally made for SCTV.
https://www.sctvguide.ca/episodes/sctv_s3.htm
The sketch doesn’t feature any of the regular SCTV cast because it was actually an edited version of a 1978 pilot for a series called “Laugh Track” that The Second City tried to get off the ground, and was simply reused when SCTV ran short of material. Created and produced by Kampmann, the pilot was written by Torokvei and Kampmann, while Andrew Alexander and Jack Rhodes were executive producers… with dialog dubbed in by Martin Short, Steve Kampmann, Peter Torokvei and Don Dickinson - all members of the Second City Toronto cast between 1977 and 79.
The full version of the pilot has since leaked online.
There’s this… possibly the earliest filmed example of movie riffing. The movies weren’t originally silent, but the dialog is muted so you only get the riff track.
I was going to mention Metropolis. Yeah, the Moroder version basically did this. But once, some friends and I did this as well.
I had a completely silent version of the film. And I mean compleatly. No soundtrack of any kind. Turns out a completely silent movie is kinda unwatchable. So we started making up the dialogue as we went along. It was in no way quality riffing material, but we made ourselves laugh a lot.
One of the three films, Nosferatu, was silent.
Some of these jokes would be made on MST today. I know I’ve heard them growl for various monsters.