Hi,
As you may have guessed, Deepdwn has no support for adding custom markdown elements, or removing the default ones. It would be very complicated to do so, and isn’t in scope for possible features in the future.
I don’t have great suggestions for your use case, but I would probably use <u>underline</u> and the existing strike through to represent your underlines and unreadable text, and then (if needed) style them as you like outside of Deepdwn, assuming you intend to host/display the digitized output somewhere.