I hate to reply to my own question, but here goes: You can't. You will only every get one byte transferred to the EXTMEM 8 bit bus when coming from the ADC using DMA.
However, it appears that you can use DRQ and NRQ even for DMA from SRAM to EXTMEM.
I DMA'd into a 2 byte buffer in SRAM. I chained the ADC DMA NRQ into DMA unit #2 DRQ which did an 8 bit transfer into EXTMEM from buffer. It's NRQ chained into DMA Unit #3, which did an 8 bit transfer into EXTMEM from buffer. Took most of a microsecond.
Using fictitious data in memory to make it easy to see on a scope, I got the results I wanted.
On to the next step.....