I'd be a little surprised if you can make a flat-fee work. Percentages are taken from pledges in part because that is part of the back-end charge from payment processors.

What happens if someone gets $1000/month in pledges and you have to pay $20 for card processing (or $35 to PayPal) but they only paid you $10/month? It seems like an disincentive to host high-earning members.