Is she a range hore or has she been bred down so that you know her lineage? I wouldn't ever breed a horse who's parentage I didn't know, because you never know what kind of defects they might throw.
I'm not trying to be the party pooper, just trying to give you some information you may not have thought of. What would you want the baby to do? Something about a Quarter horsish, mustang type build isn't going to match up well with a tall, lean, up hill WB too well. I do realize that mustangs (and I'd guess Nokotas too?) come in many shapes and sizes because they're scrub horses that basically just ended up as a melting pot for unwanted ranch horses or stallions turned loose/escaped, but I've never seen a mustang that looked like a warmblood. They've been all sorts of heights, but most are thick, sturdy type animals with 'survival' as their gene pool weed wacker.
IMO the last thing a breeding should be, is an experiment.