For start up employees, what we really need is compensation tagged with company stage (seed, series A, series B, and so on). I know private entities that maintain such a database, so it's definitely doable. If AngelList added this feature, their database would be extremely useful [0].
Also it would help to ask for the date of the negotiation and allow filtering salaries by "negotiated in the last 90 days," "negotiated in the last year", etc..
In the meantime, in SF, the best thing I and my company have found is asking around our friends... not terribly scalable :/
[0] https://angel.co/salaries