I do a lot of ruler work at my dsms, none of them are mounted on a frame, all the FMQ I do is sit down. It's a lot of fun. I like to combine ruler work with regular FMQ.
I have used both the Westalee ruler foot and the one that Janome makes to go on their convertible free motion set. It's not at all difficult and if you've got some FMQ experience you will have no problem. The only thing that takes some getting used to is estimating where to place your ruler.
There's a good class on Craftsy on using rulers, with a regular sewing machine. You can also find a fair number of youtube videos if you search on: Amy Johnson, Patsy Thompson, or Free Motion Quilting.
Rob