Currently Franky has no detection code yet, it assumes it’s always there if no other DCSG has been found.
Since Franky sits in the middle of the switched I/O space, it’s not nice to play to it without proper detection first (also I don’t want to encourage other boards to use this address).
Since the DCSG is write-only, instead we’d probably need to detect it by the VDP. This document on SMSPower may be useful: http://www.smspower.org/Development/TVTypeDetection
Was implemented last year in f72935f6caf2.
And improved in 2bac68a865d4.