@whitequark mate you absolutely smashed it
tested program-m16c with glasgow commit d7db593e8025406432dd963766c31c5660047be7
using the cnvss pin, as the target requires it
read, erase-block, erase, and program all had the expected effects
sync happily achieved at baud rates 9600, 19200, 38400, 57600 and 115200
target initially had a custom key; I appreciated the small smoothness of the default keys still being tried when --key is supplied, so no futzing after an erase
thank you!!