Kanstul experience with marching brass probably really helps here.
I have played a Flugelbone (although I think it is a funny term, a marching euphonium or E-horn is closer to the equivalent of a flugelhorn in the trombone world). Some can easily double. Others have trouble. But a traditional valve trombone is often out-of-tune and hard to play in tune. A Flugelbone is much easier. With a small mouthpiece I would suspect a trumpet player could double with a little effort. Reading bass clef in concert pitch rather than transposing a step is a little different.
Of course you could always read the trumpet charts!
M&C