Please describe the feature you'd like to see added.

If a user wants to make a chain of two transactions and sign the child without being able to get the parent in their mempool, I do not believe there is a way for this to be done natively in the PSBT RPCs. As the UTXO is not in the mempool/wallet/utxo set, it cannot be found for signing/finalization, unless injected manually.

Note that this functionality seems to exist in the raw transaction path via the "prevtx" argument for signing, via:
bitcoin-cli signrawtransactionwithwallet <tx_hex> '[{"txid": "<txid_hex>", "vout": 0, "scriptPubKey": "<spk_hex>", "amount": ""}]'

Is your feature related to a problem, if so please describe it.

No response

Describe the solution you'd like

One thought is an additional optional argument for utxoupdatepsbt to inject a UTXO from (an array of) serialized previous transactions. The PSBT Input's prevout can be found in that list, and the PSBT Input populated with the requisite UTXO.

Describe any alternatives you've considered

No response

Please leave any additional context

No response