<<Market cap is a calculation of the worth of outstanding shares of stock. Pretty useless figure on privately owned corporations.>>
I guess you never heard of "fair market value" which is how ownership of all the outstanding shares of a privately held corporation could be evaluated. Same underlying principle as market cap, smaller market; and a hypothetical predicted value instead of actual value as dictated by the market on a daily basis.
<<Last time I checked, reporting revenues was something that all corporations are required to do (public and private), and it's generally audited in some way and therefore fairly reliable.>>
You should check again, then. The standards for auditing public corporations are exhaustively detailed and minutely regulated, whereas for privately held corporations, the standard is usually "generally accepted accounting principles," a rubric within which generations of genuinely creative CPAs have flourished and occasionally fallen, and which, moreover, allows a considerable latitude for discussion as to what "generally accepted" really means.