A good coach won’t be looking for just relievers, he’ll be looking for any kind of good pitching. -THEN-, when he has a fair share of pitchers, he’ll assign certain pitchers with certain jobs. So I’d say concentrate on showing him you’re a pitcher. Then when he’s seen you, maybe you can suggest for him to make you a reliever, if that’s what you want.
And you don’t need to throw hard to make the team and/or be successful. Just show good control, movement, speed changing, etc. and you’ll be successful. Velocity certainly will help, but make sure you show those other 3 as well, and you’ll definetely have sucess.