Fortunately, the receive ISR isn't cracking packets, just calculating a checksum and passing the packet on to LWIP. I wish there were two DMA sniffers, so that the checksum could be calculated by the DMA engine(s), as that's where a lot of processor time is spent (event with a table driven CRC routine).
You can do it using PIO. I did that for emulating memory stick slave on rp2040. One PIO SM plus two dma channels with chained descriptors. XOR is achieved using any io reg you don’t need, with 0x3000 offset (manual mentions this as the XOR alias)
