Yes! The cold absolutely changes the game. I had a chance to ponder this yesterday while I was operating on Columnar Peak in -17C (1,100m vertical gain/loss, 28km round trip, strong winds on the summit.) It's an environment where taking care of yourself is a priority over taking care of the radio and practicality dictates some unusual operating practices. For example, doing everything while wearing heavy winter gloves or just throwing half the dipole on the snow when you can't get one of the end supports to stay up. Getting the operating time down to <30 minutes also helps to limit the exposure, so simplifying the set-up/tear-down process is vital. I'm encouraged that the Nokia 5110 display is already rated down to -20C (-4 F), though there are a couple of other components I worry about that I'll discuss below. So with that in mind here are a few of the things I came up with in regards to the new build:
I'm happy to hear that cylindrical cells are the right decision for batteries. As much as I'd love to be able to reliably keep a battery pack warm for these things it's often not practical, especially on 6-hour approaches or multi-day trips. To that end I'd like to experiment with a starter-battery approach to this and just overbuild the power source. For an internal battery (which I prefer to an external battery to reduce setup time on the summit) I'll start out with 4S 18650 lithium cells through a 14v regulator. This should give me enough power in cold weather to do some transmitting and get the unit to start self-warming without being too hard on the radio components. If things get really bad I might think about adding a small resistive heating element around the battery that I can just turn on in the parking lot.
Heavy gloves are a must in extreme temperatures, especially considering my history of cold-related injuries in the past (I'm still waiting for all the feeling to come back in the fingertips of my left hand.) BNC connectors are definitely easier than others, but I'm now revisiting the idea of including an EFHW tuner internally and then using a banana or RCA connector. This might be useful in parallel with a BNC connector for added flexibility in other situations.
The button and knob will also need to be easily manipulated with gloves while also being at least somewhat weatherproof. I'm still looking for the right parts, but they'll need to be able to withstand some light rain or melting snow.
In the interests of reducing the amount of time exposed to summit conditions I'm considering using a TCXO with the Si5351. With any luck this would eliminate the warm-up period and make it easier for other stations to receive the signal from a (very) compromised antenna. This is a problem I encountered with my FT817 in temperatures as mild as 5C (41f), where the receiving stations were constantly having to adjust to match my drift. Ever since I added a TCXO module I haven't had that problem again.
Due to the high winds and cold temperatures I couldn't get my logbook out to make any records. Fortunately I was able to enlist a local operator who could hear me on 20m and was able to copy down my QSO details for me (big thanks to Keith VE7GDH!) It did cause me to consider adding a tiny MP3 recording module internally so that I can retrieve the information later. There is a module like this that is also Arduino compatible, the only downside is that it would pull more than 20mA from the power supply. However, given the already low power consumption of the radio in receive, I might be able to live with it.
MP3 Module: http://www.electronics123.com/shop/product/fn-rm01-high-quality-mp3-audio-recorder-and-player-module-8227
Thanks to Allison for the advice on painting the case a dark color to facilitate some warming by the sun (at least when it's sunny!) I may also try to stitch up a transceiver cozy (or soft-case, if you will) for non-sunny days.
If anyone else has any thoughts on these topics (or others) then please feel free to jump in! I'm always looking for new ideas.