The question you posted in this thread made me curious. Actually I was going to answer you that the slow motion is always possible, because I remembered I had used it even some very old videos made by my old video cameras (in the '80s and in the 90's).
But, after having read your post, I checked some films in .mp4 in my video archive and I wasn't able to play in slow motion... So, I thought I was wrong about the possibility of slow motion in every files.
Today, I had time to check also these old recordings (saved as MPEG!) that I'm totally sure I was able to play also in slow motion (something tried the last time a couple of years ago, probably by BH 2.1.7 or 3.0.1, I'm not sure). But today it did not work. So, it's probably also the image responsible for this different behaviour and I'm curious to try, by installing in OMB, other images and try.
At the moment I tried only 4.1.022, either by its default player or by the Extplayer3 installed by ServiceApp.
So, if you have already installed other images, you can try yourself.