If none of the above apply, and you can't think of something similar and very compelling, then maybe you should just stick with normal work for pay scenarios
If there is a strong motivation to take on this kind of work I have a few suggestions about how to approach the contract side:
One last thing to keep in mind is that the old adage of you get what you pay for really is a reality most o the time. When I've been on either end of the full out free labor today million dollar payout before I die scenario the commitment has always been tenuous. I lost a guy because you he couldn't convince his wife he wasn't wasting his time, and I've blown people off because someone gave me a call and had a $500 project that needed to be done that week. A start-up that really is in the position to ask the world of you with nothing to offer really needs to make a good case, and be wonderful to work with or else it's just not going to happen.