Legally, yes.
Would it work? Probably not. Likely too little velocity.
Even the 14.5mm artillery sims that we use, tend to add too much weight to the pusher and slow the velocity too much. That's why we 3d print the whole projectile and then add the 14.5 projectile. Some have also machined lightweight aluminum pushers to hold the 14.5 projectiles.
The 50bmg spotter projos are designed to go a lot faster than 250-300fps. Not as fast as 50bmg (since they are 50-spotter) but still multiple times faster than an m203 round.
Maybe I'll pick some up and try it. But I wouldn't expect it to work well, maybe if your shooting at boulders or big steel plates