The v3.x I2C sniffer is a state machine that tracks the pins with change notice interrupts. It looks like it is in the terminal and in the binmode.
The RPxxxx is probably capable of running a modification of the existing code fairly well (16MIPs vs 125MIPs). I would guess the PIO is probably the right way to do it.
Here’s a pretty advanced project, MIT licensed, for I2C sniffing on RP2040. We could integrate a variation of that.
I also have a bookmark for an SPI sniffer update from @henrygab that I’ve yet to implement.
And don’t forget @dreg’s PS/2 sniffer.