Thanks everyone for the great feedback!
@allanhurst Thanks for your comment! I spend a lot of time these days inspecting datasheets for the Operating Temperature Range and adding to a master spreadsheet of every possible component I might utilize. Industrial (-40°C to +85°C) is an acceptable temperature range for us, and -55°C is nice when we can get it. We'll be looking and insulating the components in order to maintain a suitable ambient temperature.
@MorganS Running the batteries in series is definitely something we've considered. The thought was if we could run only in parallel, we might be able to save some money on batteries. A step-down converter however sounds like a good option to achieve good efficiency. I am looking forward to investigating this further!
@JohnRob Since purchasing and working with the Pro Trinket 3V, I've discovered I could likely do with a simpler board that is designed more for ultra-low power operation. I've had my eye on the RocketScream Mini Ultra 8 MHz , which decreases the clock speed from 12 to 8 Mhz, and has a quiescent draw of just 1.7 uA (I've gotten the Pro Trinket down to 19.7 uA). Conformal coating is definitely something we've considered, as well. Given the low-cost nature of the datalogger, we've even thought about just epoxying the whole darn thing! This winter we plan to put the components through some solid stress tests to see how they handle things.
Cheers everyone!