It's easiest to get away with electric basses because bass sound tends to be omnidirectional. It also tends to be non-piercing.
90+ percent of the time, an electric guitar will be significantly too loud for everyone except the person playing it, who thinks it's way too soft and wants to turn it up.
I have jammed with a keyboardist using a sit-down-to-it electric piano, amp, and speaker whose sound was not overpowering. I don't know if it was the nature of the instrument, the placement of the speaker, or where I was standing, but it worked in an otherwise acoustic jam.
I have also jammed with a keyboardist using a put-it-on-your-lap keyboard with only its built-in speaker. I had no problem, even standing next to him. Others did find it difficult to be next to him.
If you can trust the keyboardist to take it well if people don't like it, I'd say give it a try. By take it well, I mean TURN IT DOWN or even stop if requested, without getting his knickers in a knot. Otherwise there's always the risk that it will be a problem, and he won't perceive it or do anything about, and a bunch of folks will be upset.