I haven't looked into the details but nicely conceived. Working on similar projects, we were worried about the CPOF by using blockchain.info and I've done some work allowing interface to Electrum servers instead of bc.i. However, it can be problematic when combined with Tor, so I'm not sure I'd recommend it in this case.
This certainly is something to think about. I guess pybitcointools is using bc.i because it is instant on and only takes a page of code or so. They are doing a good job so with Tor it goes ok for me.
### Limitations
* Only small TXs will fit into the QR code.
What's the actual limit?
There should be a hard limit of about 850 characters (base58). The most I tried it so far was one prevout and three outputs. On small screens like on an very old laptop you might have to reduce the font size to be able to fit the QR code. This could be improved by using quadrants instead of half characters in the ASCII QR code symbols.