I've got a few of the Elseworld's Annuals from '94 that people keep mentioning. I'll have to go back through and re-read them before I can give you a good set-up for a campaign based off of them. However, since Justice Riders is a favorite of mine, I'll give you a pitch for it (and a brief description of the character's that appeared). By the way I don't think this is the same as the Maginficient Seven, because I don't remember that ever being mentioned in the one shot (which was written by Chuck Dixon, btw).
The Justice Riders (as they are refered to by Maxwell Lord)
Diana Prince - sheriff, often called the woman wonder
Katar Johnson - the hawk shaman (does have a winged suit)
John Jones - mysterious manhunter
Kid Flash (Wally West) - fastest gun in the west, outlaw, wrongfully accused of murdering Barry Allen
Booster Gold - maverick gambler
The Beetle - radical inventor that wears antennae
Oberon - Diana's deputy
Guy Gardner - Pinkerton tracking Wally West
Colonel Clark Kent - Dime novel writer
Maxwell Lord - mad scientist, rail baron
Lord Havok - one of Max's steampunk creations
Felix Faust - outlaw and catalyst to the story
According to the one-shot, the Justice Riders only rode together once, but what if that's not the case. The wild west is mysterious and dangerous place. It seems to be getting even more dangerous as a new gang has started consildating power. They call themselves the Apokolips Gang, led by the mysterious Boss Darkseid. Darkseid's lieutenants and his clockwork metal men (created from Maxwell Lord's blue prints) have raising all sorts of hell. One or more of the original Justice Riders decide to recreate the group and start gathering a posse to fight back.
I think the game works better if you keep the power level at 8 or lower. Pure skill, magic, and steampunk gadgetry are all good explanations for powers, though aliens have been shown to exist (and all probably refered to as outsiders or something like that. Deadlands would provide you with some good ideas. You can also use classic DC western heroes like Jonah Hex.
Around the time I picked up Justice Riders I also bought JLA: The Island of Dr. Moreau. Here's a brief outline, Lucas Carr is found adrift and goes with the Dr. Ivo to the island of Dr. H.I. Moreau (which by the way they should have changed his name to T.O. Moreau). Moreau was banned from England for his studies. He's created beast men (which are similiar to the League). He eventually takes his "children" to England to hunt Jack the Ripper and I'm not going to spoil the end, but let's just say it's not a happy one. While it's a bit ahead in the timeline of a typical western game (being Victorian), it could fit into a Justice Riders setting.
Added after original post:
Also, Steel Annual #1 is a good comic to tie in with Justice Riders. In it John Henry is a slave blacksmith who starts out as "friends" with his master's son, Arthur. Eventaully The friendship turns to jealousy as Arthur is taught how a southern gentleman should act and eventually catches John sleeping with a white woman that Arthur had been talking to. Arthur has John create him a suit of armor that he can wear during the Civil War. John Henry uses the suit and leads a slave rebellion and the ending is ambigous, as mentions several stories about what happened after the rebellion. I think John Henry and his family moved out west and he probably got caught up in some of the gang wars and such.
"I've seen angels fall from blinding heights, but you yourself are nothing so divine..."
Chris Cornell - You Know My Name
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