I went down this path a year or two ago and didn't find any success. I couldn't ever get Slingshot to work with 2.3.x Rails apps, I even thought about deploying the application on an entire OS system wrapped up into a virtual .exe using Xenocode (which is now Spoon Studio). Ultimately, I rebuilt the back-end Ruby stuff in javascript and deployed it as an AIR app, not something I'd recommend or even do again myself.
There's another thread on this topic with some other options listed in the answers and comments:
Distributing Rails Applications as Native Applications
Good luck.