Hi @T B ,
Yes, it is possible to program the STM32F401 with openOCD with MCU power supply at 2v5.
According to Table 7 of RM0368 we need to configure PSIZE with 01 (16-bit) or 00(8-bit).

PSIZE is not configured in the "stm32f4x.cfg" but configured rather in the "loader file" plugin located in ~\STM32CubeIDE\plugins\
The loader is in executable format, so not possible to customize it. I asked the development team what is the default configuration of openOCD loader for the STM32F401 in terms of PSIZE and flash clock frequency access ?
Hope it is 8-bit/20Mhz so that it is functional in all cases, otherwise a customized loader version will be requested to give guidelines how to change the openOCD project in the github.
Internal tracking ticket is "Ticket 176083" not accessible for you. I let you know for the updates.
Best regards,
Younes