I got a SoKustom swingarm bag (yeah I heard the story got before it went down) and it has a mount for the plate right on it. I will post up some pics when I get home.
You can move your palte udner the tail light with the softbrake bracket. You will be able to put it on the side, but it all depends on where it mounts. I think you can get an axle mount side mount plate. You cannot use a swing arm mount side mount though.
I ordered a Nash Swingarm Bag a while back, they are made when ordered (in USA) so it takes time depending on there back log. I like the look of the swingarm bag better, it flows nicer and you have more room then a fork, riser, kind of bag.
I have a fork tool bag, but since I got my FXST I like the look of the front end with nothing hanging on it, so I don't want the bag on there:
Instead, I'm switching to a pair of these, in chrome - they also come in black for the Night Train:
If I was riding a Night Train, I'd put one of those on each side ($40-90 each) and change out the locks to real, keyed locks ($10 and 3 minutes). I think it's far enough forward not to hit the side-mount plate.