I've actually looked into that model. You wouldn't have to have partnerships with nonprofits, you can just mail them checks (they are already set up to receive donations and you don't need permission to donate!)
The wrinkle with that concept is with pass-through donations, the donor can't claim a tax exemption unless the pass-through entity is also classified as such. The way around it is to set up a clearinghouse with a bank that authorizes the pass-through entity to write checks on behalf of members. Then when you donate your $1 a day, it goes into your bank account, and checks are written from there every month to the charities of your choice. Then you have to figure out pricing and transaction fees with the extra overhead you're introducing across sending payments to multiple nonprofits.
Maybe it's not a big deal with $1 a day, but if you want to scale to supporting bigger recurring donations people are going to want a tax exemption.
It's a good start but I think it needs to be thought out more to become a viable product.