Muskteer,
I have been to a few clubs that did allow a person to use one pistol, a rifle and shotgun. They called the catagory "working cowboy". The logic was that this was probably more in line with what an actual cowboy carried in the old west. Of course, they had to reload the pistol on the clock. As long as they competed among themselves, there is no problem. But, against everyone else, the WCs always finished last.
I also underdstand people that can't justify spending two grand or more on a full set of CASS guns. But one has to understand how complicated it already is to score a cowboy match and how many catagories there are already.
At a local club, you might find them very receptive to shooting each stage with one or two of the four guns. You also might find them receptive to using guns that would not normally be used at a cowboy shoot. But, at some point the expectation would be that you would have to get the required firearms.
As far as getting people to participate, this has not been a problem at most clubs. Cowboy action is the fastest growing organized shooting sport and most areas of the country have numerous clubs. From where I live there are five clubs within two hours and another four or five within three hours. Pretty much every club has at least 40 to 50 shooters at every match, and growing. Just like any sport, there is a fairly significant up front cost (golf has already been mentioned). After that, it is as expensive as you make it. As a matter of fact, if you go with used guns, Blackhawks can be found around 300 bucks as can a double shotgun and I have seen Marlin 94s for around 300. An enterprising pard could easily come up with the money for this. Leather can be had for cheap too, of course it won't last very long and you either eventually pay for good leather or keep buying cheap.
CAS is expensive, or as much so as you make it, but what about sporting clays where one good gun can run several thousand, or NRA BPCR where a good Sharps can easily go for 1800 to 2000, or IPSC where a race gun can run several grand, or Three gun, IDPA or any other shooting discipline. Any good AR used in three gun, high power, sniper comps or any other competition starts at a grand and goes up from there. So, in the relative scope of things, CAS isn't any more expensive than any other organized shooting sport (or golf for that matter where a set of clubs and all the fees that go with it,,,, well you get the picture).
I also understand not wanting to shoot other people's guns. That is a personal limitation that I feel similarly in. But, it is an option.
Good luck
Hank