What does the receiving machine need?
Just some way to receive the data and store it to disk. I'm using
goSerial on OS X. It has a logging option to save all received data to a file. When I send a complete disk, it gets logged in the file. I change the file extension to .img, then I can mount it in mini vMac. It's not a Disk Copy 4.2 image, but it still works in mini vMac because all the essential disk data is still there.
I was thinking more about how to transmit a disk in Disk Copy 4.2 format this morning. At first I thought it would be a piece of cake, because I could capture all the tag bytes in a 9,600 byte buffer on the Mac, and then transmit them following the disk data. But then I remembered that there is a data and tag checksum at the head of the Disk Copy 4.2 file... so that's going to be troublesome. I can't compute the checksums until I have read the entire disk.
Probably I will have to send Disk Copy 4.2 images with no checksum. After transfer, you could run a small utility on the receiving machine to generate and insert the checksum. The sending Mac can compute the checksum too, and display it on screen, so you could enter it manually if desired.
Going the opposite way,
receiving a Disk Copy 4.2 image and writing it to disk, should be somewhat easier.