The PSBT_GLOBAL_XPUB should be populated when creating PSBTs when using multisig, as offline signers such as the BitBox02 hardware wallet require it to register and retrieve a multisig account.
https://github.com/bitcoin/bitcoin/blob/817aeca57a4cc3c8b4b321dbc653bfc4a8d61b0c/src/psbt.h#L20
Would there be any obstacles in implementing this?