So, curently the custom mode options when creating a custom difficulty campaing dont allow for the adition of the honor mode ruleset, you CAN create an honor mode save and then have all your party die and then choose the option to keep playing at custom difficulty, the problem with that approuch is that, although the honor mode ruleset is enabled, you cant disable the single save option.
In conclusion, it would be great if it was possible to play with the honor mode aditions like legendary actions but without being forced to play with the single save file option.
I imagine that adding the option to change from the "default ruleset" to the "honor mode ruleset" to the base custom difficulty options would be the simplest way to achieve it.