Signing PSBTs for "proof of reserves" BIP127

It would be cool, if Electrum could sign BIP127 "proof of reserves" PSBTs that were produced externally for an Electrum wallet.
Here is the story:
https://ulrichard.ch/blog/?p=2587
and
https://ulrichard.ch/blog/index.php/2021/12/10/proof-of-reserves/

Essentially, I'm trying to sign the following PSBT:
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

for the following wallet:

{
    "addr_history": {
        "tb1q0v3yu6rxuvd90ur0g6hx69dhlemjdqz7fgzly8hzhwpp7kntg0dsfa0964": [],
        "tb1q36kysjh3mv6sgc5amzmysn3neqr230f0qjrfx8ersfkqhcf6z33ss04lp9": [],
        "tb1q53qxdzdxmnkuckg9lv7mk8djgq2nqkjgku5vlmg73y8an80k4f5qn9hr05": []
    },
    "addresses": {
        "change": [
            "tb1q7s26yhh8hmq2t6r8wa7ckpjhcugkhjshnw5hsg0m9u82kmmy5zrqdm2qc2",
            "tb1qewjj5nzrfj9fc73k392sr6pgeeyfvfy3vrxtczr4pyjwpzhez7eqtypafe",
            "tb1q36kysjh3mv6sgc5amzmysn3neqr230f0qjrfx8ersfkqhcf6z33ss04lp9"
        ],
        "receiving": [
            "tb1q6x5sfksrqg20668q0vj3phfjkrqkzx62ke3mutflxhsca4gjc48qgeusmx",
            "tb1qxna4v45xnf0j76z7cuetlpx3qdzuk2333f3anm9qymmj6u6w9drq4zzqm2",
            "tb1ql0h4d90l8wxfuccv008cl37458e8alfrn35x53rgkqzzg0f672fqms35mw"
        ]
    },
    "channel_backups": {},
    "fiat_value": {},
    "invoices": {},
    "labels": {},
    "payment_requests": {},
    "prevouts_by_scripthash": {},
    "seed_version": 33,
    "spent_outpoints": {},
    "transactions": {},
    "tx_fees": {},
    "txi": {},
    "txo": {},
    "use_encryption": false,
    "verified_tx3": {},
    "wallet_type": "2of2",
    "x1/": {
        "derivation": "m/1'",
        "pw_hash_version": 1,
        "root_fingerprint": "f6083804",
        "seed": "maximum assume mention girl puppy stereo river tourist gossip level panda life",
        "type": "bip32",
        "xprv": "Vprv1AEubNnoCJHpVL2TLDscvM6bEC7Q4ZxqkrdhxCHFb6EJtADGoSkb9cXt9tTFYqgEVSJGhM96eZmwK1G88SW2Zg1LayXACst8XFYSgS8dK8b",
        "xpub": "Vpub5gvwjnq3LfyVNo4FDsYDKkkhoDdLDu9kLAEVEiKZ1N2hieU3RqaBL79kL2wP5YNj2aL9Bbe5bFXm2BFbpStm5ixxo8SKQrCBK5DxSrJhq7k"
    },
    "x2/": {
        "derivation": "m/1'",
        "pw_hash_version": 1,
        "root_fingerprint": "b88448fb",
        "type": "bip32",
        "xprv": null,
        "xpub": "Vpub5gUii5ZKgrJs2sZfyHrKFrKdZqmek7iLGoX3jmREfdMscpyaDPLAcYm1mED7PexvD4JQPMJL3AGtMewxSjFhcUX64fKFR2bgLXAY7xPgJaX"
    }
}

Or even better this PSBT:
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

for the wallet:

{
    "addr_history": {
        "tb1q4dhcva207vq225ljqfht0xr3ydfmcjj9mcnn9rqc322fpndehhqq9d596z": [],
        "tb1q4n590s3wlqtsz4g6jzmfc8whn0fvun3vmjzpuj5740gr56jfjy2qpglrkv": [],
        "tb1q6cdpn4d373nnkvn682s37dsyhvy3y6lksdqpuzr9k3ch72mxmhzsa8ed32": []
    },
    "addresses": {
        "change": [
            "tb1qwvcpet99frsw8v6rkuymw969uwrc9vs5dpy04ru8f45xl8s6u3kq96tt6m",
            "tb1qg2cdtu7c860qgm894l65m4drzpvhskmvsx8cr6h7c72gmmeys6aqvj8cya",
            "tb1q6cdpn4d373nnkvn682s37dsyhvy3y6lksdqpuzr9k3ch72mxmhzsa8ed32"
        ],
        "receiving": [
            "tb1q4n590s3wlqtsz4g6jzmfc8whn0fvun3vmjzpuj5740gr56jfjy2qpglrkv",
            "tb1qltz8j6hlqk4yhy9svrvtkpxwtr8r7u3a6p3h807vw2l9ukenh3zql3lyr5",
            "tb1qets9smu25enexn4r66el43e473jgv42vmtwm52mc3xc9knp2wmes48skss"
        ]
    },
    "channel_backups": {},
    "fiat_value": {},
    "invoices": {},
    "labels": {},
    "payment_requests": {},
    "prevouts_by_scripthash": {},
    "seed_version": 33,
    "spent_outpoints": {},
    "transactions": {},
    "tx_fees": {},
    "txi": {},
    "txo": {},
    "use_encryption": false,
    "verified_tx3": {},
    "wallet_type": "2of2",
    "x1/": {
        "derivation": "m/48'/1'/0'/2'",
        "hw_type": "bitbox02",
        "label": "bb2 (27d81095)",
        "root_fingerprint": "27d81095",
        "soft_device_id": "27d81095",
        "type": "hardware",
        "xpub": "Vpub5mUs4UNPA6T3VAmcTWTJ2nCV2oAEFQqBNQQDH62NQNpdAMSyL2Nd3vZXF6uQfNeiCst7asUapZWM9AKmsYCK1BjUrEVhiVm9M4qnbHvDRDe"
    },
    "x2/": {
        "derivation": "m/48'/100'/0'/2'",
        "hw_type": "trezor",
        "label": "T-Rex",
        "root_fingerprint": "6bfac2d6",
        "soft_device_id": "EE735C3D13E9A60902B42A02",
        "type": "hardware",
        "xpub": "Vpub5mTgvNLEMssnVd4fezZgnDhLefVaYCb94GsjHfgrhXksbrHRbMa2AwjWX9eczB1dG19oZmEnVNCeVLWoygXQrkL8nuyAgWDxnePWUQ5fE3N"
    }
}

Of course, it would be cool, if Electrum had full support for BIP127 proof of reserves, but being able to sign would be a good first step.

Read BIP127 and the two linked proof-of-reserves explanations, then reproduce the supplied PSBT signing request in Electrum. Trace Electrum's existing PSBT signing entry point and determine what BIP127 handling is missing. Done means externally produced BIP127 PSBTs can be signed for supported Electrum wallets, with the supplied examples serving as validation cases.