Our firm has played with that concept already for number of years. However in the last few years with easy access to funding capital it begs the question why exchange equity for those items? It would seem then that this would appeal to startups only if they can't get funding, which raises another question on how viable of a startup is it?
With that said, we concluded that a capped equity swap might be acceptable. As in where the service provider would be rewarded with up to 4x to 8x their or the industry normal business rate for the services they performed. So a lawyer normally charges $150/hr. He choose to only charge $50/hr in exchange for the other $100/hr be some sort of investment. At payout time he could receive upto $800/hr for services performed. Payment could be tied to any company metric. This would be much easier to track then say % share of equity.