I have been using Microchip products with a PICKit3 programmer to program the new device firmware in the field using the "Programmer To Go" feature of MPLAB. Is there anything like this for STM32?

[Graham1904]

"Programmer To Go"

[Ozone]

That would require the programmer pod to store the firmware file, which is much easier for Pic16/18 with their little Flash (<64k).

Besides of pricey industrial multi-gang programmers, I know no low-cost JTAG pods for Cortex M with that feature.

[berendi]

A Raspberry PI with OpenOCD can talk directly to SWD, so you can use it as a programmer as long as the target voltage is 3.3V

https://learn.adafruit.com/programming-microcontrollers-using-openocd-on-raspberry-pi

If the target voltage is different, you can still use a real ST-Link with the Pi (or build a voltage converter).

Then you can connect a few buttons and status LEDs to the spare pins on the Pi for headless operation, needing just a USB charger or power bank to work.

[Roman Priesol]

There were some commercial stand-alone/offline programmers available, but unfortunately all are not available.

Evo Primer (STM32PRIM-LAB): https://www.newark.com/stmicroelectronics/stm32prim-lab/dev-board-stm32-evoprimer-monitor/dp/06X3690

Cyclone / Cyclone FX: http://www.pemicro.com/partners/index.cfm?manufacturer_id=4

But you can still buy some interesting offline programmers on aliexpress - useful for field programming, but it is not exactly what you want.

https://www.aliexpress.com/i/32892646424.html

https://www.aliexpress.com/item/4000134196087.html

https://id.aliexpress.com/item/32978703795.html