While in my case it was possible, I am not sure it would be in everyone's case. IMO, if someone pledged for something and then was told it couldn't be delivered without paying more money, in that case I believe that it would be reasonable to ask for a refund. IANAL but there is an expectation that you were going to get the product you had pledged for (or something equivalent to it) irregardless of the Indiegogo T&C and I'd go straight to Paypal or my bank as appropriate if I was refused one in that case. The other case, which is connected with a post I made earlier regarding my concerns about the reason for the headphone campaigns, is if they were accepting pledges for the campaign knowing that they were unable to deliver the goods because, for example, they didn't have enough money to manufacture them. Again IANAL but that would surely be illegal.
The usual note goes here that I'm posting this as a member and that this is all theoretical.