Well, there is an alternative to "Working for free".
You could try to find something which has monetary value for them and try to create it by yourself and selling it to them.
It doesn't have to be a technological solution it could be any business related thing - potential customer database, sales leads etc.. (I am not sure what you are good at so...)
Basically, if you have something of value you could always try and sell it to someone - and if it is of value someone will pay for it or at least might consider your contributions worth more than what it costs to have you there.
You could try to find something which has monetary value for them and try to create it by yourself and selling it to them.
It doesn't have to be a technological solution it could be any business related thing - potential customer database, sales leads etc.. (I am not sure what you are good at so...)
Basically, if you have something of value you could always try and sell it to someone - and if it is of value someone will pay for it or at least might consider your contributions worth more than what it costs to have you there.