Is it possible to adapt a NEMA 14-50R to a NEMA 6-20P or NEMA 6-30P appliance?
Nothing easier.  You're already on a 30A breaker, just change the receptacle to L6-30R.  

Don't even bother with an adapter cable.   

Just cap off the neutral inside the junction box, you won't need it.  

However the breaker must match the receptacle.  If you change to a NEMA 6-20R receptacle, you'd need to change the breaker to 20A.