When I went to the quilt show in Des Moines last year I tried the sit down quilting machines (HQ Sweet 16 and Baby Lock Tiara - same machine). The HQ dealer had a long arm ruler on the HQ table and I asked if I could try it, and I did! Now, I was not wearing gloves like I normally would when FMQ on a domestic, so that made the whole experience a little more difficult, but I DID use the rule and I DID get *some* straight lines. It WILL take practice, just like FMQ but it can be done.
I have no idea which ruler it was, but I know it was a ruler made for a long arm because it was much thicker than a rotary cutter ruler. It was the length of my palm, and about 1/2 as wide. I positioned most of my left hand (ring finger to thumb) on the ruler with fingers pointing to the needle and the flat edge from pinky to wrist was off the ruler and on the quilt.