Hmm... looking at the doc again, I guess it does say that DMA is only 68000 to SH2. So I guess you need some other way to send data the other direction.
You should be able to transfer easily thru the comm port. It just requires a little bit of sync. Fahrenheit uses this method to send 10 to 20 KB frames, 12 bytes at a time, from Word RAM to the SH2. Transfering in the other direction shouldn't be a problem.