And if you do succeed at getting 100 respondants, you should consider offering a small payment for involvement.
Good idea, I was thinking about that, too but don't you think it will skew the results towards people with the least amount of Bitcoin? (they would be more likely to chase a small payment) Whereas their unrecoverable passwords for Bitcoin wallets will probably be not important enough.
Personally, I don't think it would. But factually, it may. Here's what you can do. Screen people for their participation and make "personal BTC holdings" a question in that screening. Or if people experienced with unrecoverable passwords is your target for surveys you may consider screening based on length of time using bitcoin (veterans vs. newbies), or transaction volume, or wallet volume.