It might be possible to make it work on the Electron, with some limitations.
The problem is that the sector size of 128K is set in hardware on the STM32F205. In order to write to flash you need to erase it first, and you can only erase to sector boundaries. So you can’t split a sector in half and use it for two different things unless you plan to only ever write both at the same time.
So you basically need to have a 256K extended application and also a 256K OTA, they can’t be 192K each. Now there appears to be enough room if you take over the OTA backup and decompress region, but I’ve never tried it.
At minimum it would require changes to system firmware, and of course the build tools.