If we're talking about incentive alignment being the primary driver, I personally don't believe it's impossible to justify taking an equity cut, if possible and reasonable (10-20%), with the mutual understanding that the outcome is going to be much better if we help with the beginning of the process. Building a reputation for being really awesome would be challenging - people would want to know that it's worth the fee. Two other big blockers:

1) Multiple levels of taxation

2) Startups will be wary of having random third parties on their cap table

There's probably other stuff too. But I don't think it's impossible, just very hard. There are other ethical concerns from this side as well - hollywood talent agents don't always have the best persona. It's part of why we're avoiding the "Agent" moniker at http://offerletter.io and branding ourselves as Advisors instead.

This is a long ways off, of course - there's a lot of work to be done even in at the narrow step of the funnel we're targeting right now.