I think that you confusion is that the table on Page 2-11 of the 1794-ASB user manual does not list the 1756-DHRIO as a scanner.
It shows the 1747-SN, the PLC-5, the PLC-5/250 and the PLC-3.
That does not mean the 1756-DHRIO and 1794-ASB are incompatible. The DHRIO simply isn't listed in that old manual.
The 1756-DHRIO has two channels that can be configured either for DH+ networking or as RIO Scanners.
Each channel configured as a RIO scanner can support up to 32 physical devices, with up to 32 "logical rack" numbers.
The Rack Numbers are in Octal addressing, and the 1756-DHRIO supports the full range of 00 through 77 (like the PLC-3 did).
If you have six 1794-ASB modules, the simplest thing to do is set them up as "rack" numbers 00, 01, 02, 03, 04, 05.