While it takes some work, it's definately possible to extend the 7" minilathes this way. I know of several other who have also extended their lathes this way with good results. The concept is pretty straighforward & as always the devil is in the details. I think the most common stumbling block is the variation between beds where the gibs ride. There are a couple ways around that, one would be to mill or file a uniform surface on one bed to match the other. I chose to go the adjustable gib route, it's a less perfect solution but it had the advantage of being simple. I simply adjust the gib to allow it to ride best over both halves. It's not like I'm doing fine pitch threading that far from the headstock.
a few folks have gone with a much shorter extension ( in the 4"-6" range ) and have gotten good results. That had actually been my original idea, to just add a small extension where I could "park" the tailstock and gain some usable real-estate. Once I started playing around with it I decided to add some significant length and the website details what followed. The same idea would still apply though, take one of the LMS extended beds, combine it with the setback tailstock and then add a small extension to park the tailstock on and you'd have something in the ballpark of a 7x17. The bed kit & tailstock alone get you to a 7x15 & extensions only add to that.
Dave
www.broncosaurus.net