How to use ceramic glaze to waterproof an earthen tank?
Glaze is essentially glass that is formulated to bond with the supporting clay body such that it's physical properties such as coefficient of expansion are similar. Since your "clay" body is unfired earth of varying composition, even if you were to succeed in applying it, it will quickly crack and spall off.

You also will never get the consistent temperature control required for a homogeneous glass surface. A good part will be either over or under fired and not water proof. Also, pure glass fires at very high temperatures. To produce glaze that fires at lower temperatures, a flux must be added. Traditionally, low fire fluxes have been various lead compounds, which have been proven unsafe for food and water contact. I believe alternate food safe fluxes have been developed, but their functional temperature range is very restricted, so may not work for the application that you envision.

I commend your out of the box thinking, but this idea is just not practical. Keep the ideas coming though, you're bound to hit a good one eventually!