That is uhm, very weird.
That is only half the rent, the other one is merging the bootloader.hex when creating the firmware hex. Both of this requires changes in the Python code of platform-atmelmegaavr (or an addition of a extra_script) and some knowledge of PlatformIO and SCOns.
For example, in arduino.py, at e.g. line 76, you would have to add something like
if build_core == "MegaCoreX":
env.Append(LINKFLAGS=["-Wl,--section-start=.text=0x200"])
and then this would have to be addapted to include the bootloader hex for the board somehow. It’s not super trivial.
Definitely an issue should be opened at Issues · platformio/platform-atmelmegaavr · GitHub