I'll take a little advice, if you have some. How can a guy help bring out a shy girl's inner domme? The girl is interested on some level but very unsure.

Communicate!

Yes, it certainly sounds as if you have already, or else you wouldn't know that she's interested on some level. However, explore that level a bit. Has she ever tied you up? Not a bad way to get her in that mode. Once you're helpless, it might inspire some of that domme spirit. Grab a pair of cuffs if ropes aren't your thing and let her feel that surge of sexy power as she binds you and does what she wants to your waiting body.

But here's the thing: DON'T PRESSURE HER. No "come one, why not?", no "it's not a big deal, just do it". Make it known that you find this idea hot, that it's up to her how far she wants to take it, and that you're okay with her being that domme. Then communicate before, during and after. Little by little, you'll start to develop that taste for domme. The rest is up to her and how she feels about it.

But just be open, be ready and be supportive.