Consider the following approach. Talk to the founders of the consulting agency again. Say something like the following.
I am currently in a high risk, potentially high reward situation with a start-up. I love the technologies and the creative nature of the work I am doing with start-up X. I know you are looking for additional employees right now, but since you are growing quickly I suspect that you will be looking again in 6 months and maybe again a year from now. I would love to sit down with you once every 3 or 4 months over a cup of coffee and share some of what I am learning and hear about how things are going with your consulting agency. If things start to fall apart at the start-up or it becomes clear that transitioning to a FT employee is not really going to happen we can talk about whether you folks have an opportunity at that time.
And, by the way can you give me a list of the 3 or 4 biggest gaps in my skill-set? One's that if I spent some effort learning would make me even more valuable to your company at some point in the future?
I suspect that this would make you stand-out to the consulting agency and may help you hedge your bets on a risky venture.