If the money would be need to be paid upfront from the charity pot and then reimbursed from the attending members, could you not just re-word what the payment is. Have it as a minimum donation to the e-goat charity fund rather than a non returnable deposit. I'm sure that anyone who pays to attend and then, for whatever reason has to back out, wouldn't mind a small donation to our charity fund.
As treasurer, trust me that the charity pot will in no way be touched to fund this event, not even on a temporary basis. If we don't think we have enough interest to host it, with deposits, then it is unlikely to go ahead.
There is however, very little to say right now because it is in the very early planning stages.