Clay,
After paying about $8/ft for a commercial trench style curb, we opted to buy our own machine and play with it a bit. We found it was very successful and has paid for itself very quickly. We bought one from Northern Tool (
http://www.northerntool.com/shop/tools/product_200356921_200356921) and modified the form to make it a taller curb by adding 2" of flat steel stock to either side of the form. As you are pouring, you keep the bottom of the inside edge of the form on top of the path, with the outside edge about 2" above grade. This kind of gives you a wider footer that the concrete will overflow and pour outside the curb width. Similar to the commercial style trench curb machines they use on roads. I would not pour it directly on top of an existing path and there would be nothing to hold it in place.
We will dig down next to the path about 2-3" and wide enough to accommodate the curbing machine itself. This area has to be perfectly level and smooth or else you'll be constantly adjusting the height of the machine. We then mix our own batches in our concrete mixer using 3 bags of quickcrete, 5 scoops of Portland Cement and 7 scoops of sand. You want the mixture just wet enough that everything is mixed, but dry enough that it will hold a form. If it is to wet, the curb will simply begin to slump as soon as it is poured out. The vibration of the machine doesn't help with this either. Too dry, and it won't stick together. If you can get a few bags of fiber mesh to throw in each batch, that's even better, but we have yet to do this. You could also get away with driving rebar into the curb trench with about 3" sticking out to stabilize it. We don't score or cut joints in our curb as I feel that leads to a breaking point. We have had some in for about 4 years now with no issues, even with driving tractors, carts and fertilizer trucks over them. It takes some practice, but with the right mix and operator, it turns out quite successful.
With this method, we can do about 250' a day, with 4 people - two mixing and loading, one finishing and one pouring. That's prepped, poured and backfilled.
I couldn't answer your question about frost, but could see it as an issue.
If you want some pictures of the form, or curbs, let me know.