On how to sign the TX, it would be an SD card PSBT dance between the internet-connected Sparrow computer and the non-internet-connected Tails+Electrum computer, much like the previous PSBT SD card swapping between Sparrow and a Coldcard. Between computers, a separate USB stick could also be used in place of an SD card, so that would be the "air gapped" part. Or I guess QR codes would even work with the webcams, though clumsy on laptops!
On how to sign the TX, it would be an SD card PSBT dance between the internet-connected Sparrow computer and the non-internet-connected Tails+Electrum computer, much like the previous PSBT SD card swapping between Sparrow and a Coldcard. Between computers, a separate USB stick could also be used in place of an SD card, so that would be the "air gapped" part. Or I guess QR codes would even work with the webcams, though clumsy on laptops!