I have a Trangia kettle that doesn't fit my 27. I assume the previous owner bought the one for the 25 by mistake. It doesn't fit inside like it's supposed to, but isn't quite large enough to fit in the frying pan position either. We use it on the suitcase stove for car-camping, as it's a good size for a couple of cups worth.
It wouldn't need to be much larger diameter to be stable in the frying pan position though, so maybe it's possible?
Not sure that that's actually desirable though, as surely efficiency suffers. Maybe if you're carrying both sizes of Trangia in a group or something...