I note that using the struct module, a user can specify if number items
of the format are to be interpreted as either big or small endian.
However the format specifier can only accept one indicator character at
its first character to apply for all number items in it. e.g:

"<HI" specifies that both the unsigned short and unsigned int are to be
interpreted as little endian values.

It would be nice to be able to instead do "<>HI" where the first '<'
character indicates the default endianess (for compatibility purposes)
and then the second '>' character indicates that the unsigned short
should be interpreted as big endian. The unsigned int defaults to small
endian as per the first character.

I don't see any reasonale reason for mixing endianess inside a struct, and don't know of any real-life protocol using per-item endianess.
Unless someone comes up with a good reason for doing so, and since no one else seems to be interested by this issue, I'd suggest to close it.

I agree that this seems like something that would rarely be needed.  If it's really necessary, it's possible to break up the format and do the packing/unpacking in fixed-endian pieces.

FWIW, I concur with Mark and Charles-Francois.