I'd think that a cache of known keys (trust on first use), synchronized across devices using browser sync, would be quite effective at protecting against MITM attacks as long as your first access to a LAN server (on any computer you have) is safe. Additionally there are orders of magnitude less attackers to a computer on a LAN than an Internet-facing machine. In fact, unless I have untrusted boxes on my network or targeted attacks, I actually feel safe hosting HTTP services (and I'm doing so right now, more or less safely), and if someone is performing targeted attacks on my network they could just break in and clone my hard drives without having to bother MITMing my connections (I value the ability to access my data across dual-boots more than I value disk encryption through Bitlocker or Linux-specific methods).