A forum I was a member of died. There was a thread on there for a GC / Wii compression tool called, I believe, nNASOS. The original tool was NASOS / Not Another Shrinker or Scrubber. I believe this was made by Redump moderator LedZeppelin68. Using that concept as a base, a user by the name of "edc" wrote their own implementation of the "not shrink" concept. Both tools implement a pseudo random-data generator that matches what Nintendo used for the GameCube and Wii. As such, a 1.3 GB GameCube game or a 4.5 GB Wii game (single layer) could be shrunk in size, with the random data filling up the disc regenerated whenever the user wants. These dumps could be regenerated into Redump-compliant dumps.
Any help tracking down these programs or the people who made them would be helpful. I suspect LedZeppelin68 will be easy to track down but I don't know if edc was a part of any other communities. LedZeppelin68 appears to have a new version of their program on GitHub.
Was there a third user in the nNASOS thread trying to implement their own version? I can't remember.
On GBAtemp there is a thread by nanook with another tool, SWiiT.
It appears to support .dec files but I have not used it.