Here's my main concern: Without an incentive to write on bills, the only people who are going to do it are those who want to spread the word of bitcoin, and use their own bitcoins and time to do so. That group of people isn't very large, and already has plenty of (potentially better) means of doing this (i.e., physical coins, etc).
So, offer an incentive to people who write on bills, and this project will take off. Don't offer an incentive, and the project isn't going to go anywhere. It's really as simple as that. Most people are greedy enough that they don't want to spend time and their own money on donating towards this. Subsidize the time of those people with those who DO want to donate, by offering incentives to the greedy ones using the donations of the generous ones. And advertisements.