It should work at least somewhat if the bar has multiple built-in speakers. I believe they achieve their results aiming the 'surround' outputs out at the walls in a tightly focused pattern and letting it reflect at an angle to the back of the room, such that the sound arriving at the listener's ears actually does seem to have originated from behind and not in front. They may also use psychoacoustic trickery to further differentiate the rear channels from the front, such as tweaking the phase and EQ to more closely match the way we naturally tend to perceive sounds coming from behind or to the side of us.
I haven't spent enough time listening to soundbars to comment on how effective they are, nor do I know enough about the underlying technology to give a more detailed explanation. But my gut reaction is that a bar with at least five built-in speakers should do a reasonably good job of approximating what the track should sound like, provided it is used in a small room so that the reflectivity effect can be properly employed. If it has a lesser number of speakers, then it is downmixing and this is definitely to be avoided, especially if it then employs some kind of reverb or delay processing to create a fake surround out of it. Given the way I made the 5.1, I'm pretty sure this would sound bad.
But in any case, it's absolutely worthwhile to hook up a subwoofer, because the bass does add a lot to the film experience. It's probably more bass than the original 70mm track of ESB actually had, but I kept it as strong as I did because the SE bass just plain sounded good. I spent a lot of time balancing it into the 1993 mix and making sure it felt like it belonged there, and didn't stick out as some kind of unnecessary hamfisted addition. Every single instance of bass has been edited in some way, some of them quite extensively; and some of them I created myself to fill in gaps where the SE ought to have had bass but didn't. There are probably more things I could do to improve it even further, and I have some ideas on a possible alternate version that wouldn't use the SE mix as a source at all; but for now I'm very happy with it. I'm glad to hear others are enjoying it, too.
As has been said, the 35mm stereo mix sounds very good, so if you don't have a 5.1 system, or want to be as strictly authentic as is currently possible, you still have an excellent soundtrack to listen to. It it sourced primarily from the 1989 SWE laserdisc release, with various defects cleaned up and the dialog level matched to the 1993 version. It is a considerably more robust track than the stereo mix of the first film, with greater dynamics and a stronger EQ—I chose the SWE version because it was the most powerful rendition of this mix found in Schorman13's laserdisc audio archive, and also because the quality of analog-to-digital conversion was improved over the earlier digital releases. Unfortunately, it was also more noisy and had more errors (dropouts, clicks, etc), but I was able to eliminate the most obvious instances. (It's possible there may be some I haven't noticed, but if so I'll fix them at a later date.) I do like the 5.1 track a great deal—aside from the lack of true discrete channels, it's exactly how I would make the film sound if I were in charge of mastering it for an official home video release—but I can also watch the movie with the stereo mix and enjoy it just as much.