How the hell are you gonna dump a RAM cart with a ROM dumper? There is no ROM to dump!
So let's see; in the case of M3 and Supercard, if being ran from SD / CF space, that means the DS is talking with the card through Slot-2, and then you're also gonna need to do RAM writes through Slot-2 as well? But it's already being used for another purpose!
As for the case of EZ or G6 carts, the rom is ran from it's flashable rom space, so it really has no other (ahem) large enough RAM to use?
Did anybody understand that? If not, try this: Run a DS rom from your favorite backup device. Then while the game is running, yank out the device from Slot-2. Oh hey guess what happened? So if the rom is currently using Slot-2 already, it doesn't seem quite likely that it'd do another process like write data to RAM in Slot-2 at the same time.