Yes, 64DD images have been dumped. I don't know how many, and I'm fairly certain that they're not open to the public (last I heard). Though, I don't know if that's quite what you were asking. HCS is/was working on ripping F-Zero X 64DD Expansion to USF, but last I heard there was very little progress on that. The biggest holdup, to my understanding, is that HCS said that to rip F-Zero X expansion, he'd need to replace the songs/files in the F-Zero X N64 ROM with those in the 64DD image. And, since HCS can't easily find the files in the ROM or in the 64DD image, it's proving quite a challange.
Basically, that's about all the info. I have regarding 64DD images. Hope that answers your question, a little. Mouser X out.