Hi WarrenB. According to the Tokyo Pen Shop Refill Matrix, Uni refills are too loose for a Zebra Prefill. Having both pens, I gave it a try.
The Uni refill goes into the Zebra hole, and for my limited test, it stayed while I wrote a bit with it, clicked another color and did that again. It does pull out easier than Zebra refills do.
Now would they pop loose while traveling with you? Maybe, maybe not. So you would have to reinstall them. But maybe they stay put.
I really like Uni colors. Zebra has three (I think) refill styles, but as for the .4 Sarasa that came with mine, Im not as pleased with them as other brands. It pulls out like it fits pretty snugly.
Yikes, I reread your post, and realized I did the test in reverse. Just tested Zebra in a 3-body Uni, and in went into the slot. It clicks out well, writes well, clicks off well.
Edited by Misfit, 30 January 2019 - 04:43.