This whole CAPTCHA nonsense might, of course, be (in part) IP address related. At least there’s https://accounts.google.com/DisplayUnlockCaptcha. Not sure how that is actually suppose to work, no useful documentation on it either (surprise!). Anyway, I decided to work on proxy support in the setup wizard.
HTTP(S) is fairly unproblematic. Just a ton of boilerplate code, but you can easily switch between proxy hosts on a per request basis (e.g. if you want to bypass geo blocking). What’s giving me headaches is SOCKS5 (Tor, SSH tunnel). For this one you basically have to reboot the entire network code when switching hosts (killing transfers). I guess, I’ll postpone that for now, though SSH tunneling could be real handy.