New riders is an interesting angle, though cost is a big factor. To handle an MX track well, a quad rider needs that initial $8,000 investment in a (new) machine plus suspension and a few motor mods. Unlike bikes, it's a little more costly to hit the MX track as opposed to the trails on a quad. So cost is an obstacle that needs to be overcome.
This is an issue that has evolved to the problem it is today. When I started racing (again) in 1998, you could actually be competitive with quads on a vintage 3 wheeler, or a stocker quad. What happened was the "woodsman" supposed beginner quad class meant for trailriders became filled with sandbaggers running highly modified quads that would have a been a trailriding joke. Suitable beginner classes are a big obstacle for people getting started in racing. I won't even get into the mini quads right now, as my own kid was stuck in those "buy a win" classes. This could be as simple as redefining woodsman or quad D or whatever you want to call "beginner quad" as stock frame, axle, and a-rms with no big bores or strokers as it should have been in the beginning. You can further help by making it a non-points class, and even a non-sanctioned class. This has been a valuable tool in bringing 3 wheelers to CRA events as they only required CRA card for the CRA 3 Wheeler series, and has resulted in most of our guys getting them anyways.
They key to remember is that the practice track is the gateway to the race addiction, though. Race addicts will spend lots of money. The "I wonder what its like" guys won't. We need to get them to the practice track as painlesly as possible to develop that addiction.
Smith Rd has been mentioned a few times in here as "tried and failed", and I actually think they are the ones to watch. Their first quad practice was a failure after little advance promotion, and most of it on bike forums full of quad haters. Their 2nd one, they decided to go with a prepay deal, and that got about 25, which they decided was sufficient to be a success. That night they made the horrific mistake of announcing there would then be a weekly quad practice. Translation - Now that you guys have busted your butt and twisted arms to make this practice a success, we expect all of you to commit to doing it EVERY WEEK or we will shut it down. That ddnt go over well, and the next one was a flop. They then regrouped and made YET another effort to run quads the last few hours of a SUNDAY bike practice. Bikes could stay and ride the track with them. They had maybe 15 quads (w/trikes). Why are they the ones to watch? They learned to integrate the low quad turnouts into their existing bike practice without much added monetary risk or cost. A Sunday has enough hours to accomodate both groups. If quad turnout is low, it just means more ride time for bikes. They also showed that they are serious and committed to making it happen by not throwing in the towel, and trying new ideas when other methods didn't work. They learned not to saturate their own market, and are talking about a few special weekend practices that include quads that wil be announed well in advance, while continuing their traditional weekday bike practices. They started promoting on facebook instead of relying on a bike forum with more than its share of quad haters to get the info out. They are seeing the advantages of having WEEKEND practices. (I always thought the idea of routing racers to certain tracks by not having weekend practices was a dumb idea.) I believe Smith Road will be successful, and that as faith in their committment slowly gets re-established with quad riders, the numbers will continuously increase over time. This will eventually bring more participants to their races because its sucks trying to learn to ride an MX track by signing up for races.