These unique devices are key to compact home garage gyms. I found this photo online after researching leg curl and leg extension attachments for power racks. Even with buying a combo 'leg extension / leg curl machine' it would still take up a good bit of space in a small home gym.
So the logical answer would be to buy a special rack attachment that would enable the lifter to perform extensions and leg curls for the legs. The device pictured I'm not quite sure if its for leg extensions or leg curls but it would be cool if something like this could be designed to be used for both muscle groups.
Having the ability to "easily" add and remove such a training device to and from your rack would be fantastic and save a lot of training space (and money!). Unfortunately, gym companies don't seem to be thinking about equipment like this and I believe there's maybe only one or two companies designing such devices. One such gym company is Sorinex.
You do have companies like Ironmaster who have designed a leg extension / leg curl attachment to be used on weight benches which is great but if you don't currently use an Ironmaster weight bench then it means buying "another" weight bench (which takes up more storage space) OR... you do have the option of perhaps replacing your current bench and sticking with only Ironmaster.
Let's hope more big named companies can start catering for the "Home Gym User" and design more clever, heavy duty, compact training equipment.