I'm late to this party, but..... I have just returned from the Lancaster AQS show and they do have the ruler foot for the Bernina long arm and it will fit domestic Berninas. It looks well made and I'm excited to finally have an option for a proper ruler foot for my Bernina friends to use ruler work for smooth lines and curves while free motion quilting.
Since I pretty much got the whole free motion ruler work thing started, I've checked out all the ruler feet on the market. My hands down favorite is the Janome combination that results in what I call the ruler foot. But I'm a Janome user so it's an easy thing. I don't recommend the Parrs Reel Ruler foot as it has been made an odd size and won't work well with other rulers to give 1/4 and 1/2 inch spacings. Plus its a hopping style foot with the cheapo bar over the needle screw. Not much height adjustability either. The Westalee ruler foot is nice if you can't use the Janome foot. If you can use the Janome ruler foot combo, it's a toss up. The Janome foot adjusts better for batting thickness and is rounded on the base of the foot so that the fabric doesn't get 'pushed' upon occasion with a bulky quilt. But the Westalee foot has better visibility around the foot with its simpler foot attachment. The Westalee ruler is probably cheaper even with shipping than buying the whole combination of the Janome convertible free motion foot set and the frame quilting foot set. Both still require the adapter shank to work with Bernina. I talked to a Brother dealer at the show and the Janome foot can be used on Brother machines, as on several other machine brands. (No, I don't work for Janome.)
So visit my blog if you'd like to learn more on ruler work on sewing machines.
Amy's Free Motion Quilting Adventures
Additionally, I don't recommend doing ruler work with a standard free motion foot. Way too easy for the ruler to slip over the foot and break a needle and possible throw your machine out of timing. Nor do I recommend using thinner rulers for ruler work as they could slip under the foot with the same results.