That’s fair, there’s always the line between “doing what’s right”, and “doing experimental stuff because I’d really like to use PlatformIO instead of the Arduino editor because it makes me happier”. 
Ultimately, I figured it out. The extra image wasn’t the issue - that only breaks booting into USB mode for drag-and-dropping firmwares, which I don’t use. I ended up comparing the verbose outputs for both tools compiling, and noticed differences in flags. -DARDUINO_SERIAL_PORT=1 is probably the most significant, but just for the record, I’m sharing my platformio.ini that works for me, just in case anyone else is looking:
[env:MagTag]
platform = espressif32
platform_packages =
toolchain-xtensa32s2
framework-arduinoespressif32@https://github.com/espressif/arduino-esp32.git#2.0.0-alpha1
framework = arduino
board = esp32dev
board_build.mcu = esp32s2
board_build.partitions = huge_app.csv
build_unflags =
-DARDUINO_ESP32_DEV
-DARDUINO_VARIANT="esp32"
build_flags =
-DARDUINO_MAGTAG29_ESP32S2
-DARDUINO_SERIAL_PORT=1
-DARDUINO_VARIANT="adafruit_magtag29_esp32s2"
-DBOARD_HAS_PSRAM
monitor_speed = 115200
And with that, I’ll patiently wait until there is official support. Thanks for your help!