You CAN test out/teach her to neckrein in a snaffle. All you do is just use your reins like you were riding in a curb and use your legs to steer. That's what I did to Lacey, I trained her to neck rein by just using my legs and laying the rein against her neck when I wanted to turn and it took her about a month of only riding once a week to catch on completely. She was obviously still "green" at it by that point but she was neckreining reliably. Now that it's been a year, she's perfect. I hardly ever have to direct rein, even when she's excited.
I could put her in a curb if I wanted to, but since she can't have a snaffle in her mouth becuase of her melanomas, I'm not sure if she could have a curb either. I might decide to try at some point since Arabs+curbs=DROOL, but currently, I just neckrein her in my bitless shindig and she does fine.
This is the kind of curb I plan to get if/when I get one (smrobs recommended, lol): SS Hinged Ported Roller Futurity Bit 5in - Horse.com
And, this is THE bit smrobs recommends to people and uses herself (can you tell that smrobs is my cowgirl hero? Hahaha): NP Sweet Iron Loose Cheek Western Curb Bit 5in - Statelinetack.com