It is highly likely that the IRS would treat any instrument described as "Like a stock option, except minus the tax treatment for stock options" as "a stock opt

It is highly likely that the IRS would treat any instrument described as "Like a stock option, except minus the tax treatment for stock options" as "a stock option." The magic words to ask your accountant about are "substance over form doctrine." One of many consequences: an informal agreement, backed by paper or otherwise, to give you compensation in event of an acquisition, where that agreement survives your departure from the firm, is taxable at ordinary income rates on at its fair-market value. This income is realized in advance of the eventual acquisition/sale. That's why startupers care so much about their 83(b) elections, because otherwise that landmine bankrupts people.