I dont know if anyone else has tried this but I had a need to make GPO make an Euph. sound with the range. As everyone might knowhue tubas range limits it to a high G which is unacceptable. Whats more there are issues with the sound not having enough edge in the lower range.
In fooling around with GPO a little bit I found that if you assign either solo tuba 1 or 2 to a channel, lets say channel 1 and assign the equivelant horn or trombone to the same channel, it will mimic the sound and range of a euphonium.
The horn works best because the timbre is naturally very similar to that of a tuba<due to them being the same type of conical brass instrument, also due to the fact that they are of similar size>, thus allowing you to transition between the different elements of the range more seamlessly. Adjusting the individual volume of each sample is a must to get the sound that you want, but I found that setting the tuba to -15.1dB and the horn to -12.4 dB gets the closest sound when using solo tuba 1 and solo horn 1.
Its not perfect, but it will be passable for band and brass choir arrangements until the marching band and concert band libraries are released.