Update: yesterday I got the first version of this idea actually working. The hardware set laid out on the back seat of my car is exactly like I planned in my original post: a sysmoBTS 1002, a Raspberry Pi model 3B+, an LTE modem in USB stick form factor plugged into the RPi, and an Ethernet cable connecting the RPi to the sysmoBTS. The RPi needs 5V power and takes it in via a micro-USB power input connector; sysmoBTS 1002 needs the same 5V, but has a barrel-type power input connector. I got a cable that goes from USB-A plug to the right barrel connector for the sysmoBTS, plus a regular USB A to micro-B cable to power the RPi - so now I only need two USB-A power ports for the whole assembly. For my initial tests I used an adapter that provides USB-A 5V from the car's "cigarette lighter" power port - in my car this port remains powered even when the engine is off, so I could lock my car and walk away with the setup still powered. This method works, but is quite dangerous in that I could run down the car starter battery and leave myself stranded if I run it for too long.
In terms of software and logical functionality, the RPi connects to the Internet via LTE, establishes a WireGuard tunnel with one of my servers acting as the MSC, and then acts as the BSC between this distant MSC and the sysmoBTS next to it. The Linux distro I run on this RPi is Devuan - the moral principle of not using systemd is important to me! Devuan still comes with NetworkManager and ModemManager though, with nmcli and nmtui configuration tools, so I used those tools to configure automatic bring-up of LTE mobile Internet connection and a static IP assignment on the local Ethernet that connects to the sysmoBTS and also allows management access for me. My own custom rc.local script then configures WireGuard and starts osmo-bsc and osmo-mgw processes. Those Osmocom components were built from source: Osmocom provides prebuilt binaries for systemd-based distros, but not for more principled Devuan. OsmoBSC running on the car-resident RPi connects to OsmoSTP on my MSC server, with SCTP connection running inside the WireGuard tunnel over public Internet accessed via LTE. RTP streams for voice calls also run inside the same WireGuard tunnel.
Aside from the danger of running down the car starter battery if I leave this setup on for too long with the car engine off, system performance is mostly OK in the initial round of tests: I can walk a little bit away from my parked car and still have usable signal, and voice quality was mostly OK in my limited tests so far. Going forward, I plan to buy a high-capacity USB power bank (60 Ah is the biggest I found) and use it as the power source for this back seat BTS instead of the car starter battery - then I will be able to exercise this system a little more thoroughly without risk of stranding myself. To be continued...