IA> Unfortunately [IBM's] BM needs the 'I13X' in MBR passed
IA> to it, [...]

.... which is rather silly, considering that it could simply perform the
necessary enquiries directly itself.

IA> If there was some way to permanently set [IBM's] BM to use
IA> the Logical Block numbers instead, we would not even need
IA> the I13X in MBR I imagine.

The silly "I13X" flag in memory _is_ the way that one sets IBM's Boot Manager
to use logical block addressing.

The correct fix to the problem would be a Boot Manager that didn't expect the
MBR to test for the presence of INT 13h extensions, but that did so itself
directly.