I think it depends what you mean by "tracker." If you mean a financial instrument that you can get returns out of, probably not (this is by design, and almost certainly a Good Thing). If you mean a thermometer for the "health" of these types of companies, you're probably going to end up looking a lot at employee/consumer sentiment. Most people in the US are employed by these small private companies so in aggregate they are probably the best source for whether or not things are going in the right direction.
Payrolls gives the labour side of the equation, and captures listed / unlisted / self employeed. Tax receipts probably a pretty decent metric as well. All very much secondary data that would indicate the real number (but its how they calculate gdp etc numbers up in the first place).
Also captures government payrolls, which is probably equivalent in activity in the economy to small businesses. (? cant be bothered to look up where it would rank properly)
