I have a design with a non user accesible low voltage section. Is mandatory to do mains isolation to pass succesfully UL/CE/CSA despite my low voltage section is not accesible for user?
No, many products are on the market (especially inexpensive appliances) that do not have isolation and are listed.
It may not end up being economical because all the points where the user could contact the circuitry (for example, a membrane keyboard that could become brittle and split open) will have to be examined for potential hazards.
Probably not if this low voltage section is properly insulated from the user, and reasonable steps have been taken to mitigate likely failure mechanisms. Basically, if it's not isolated, it will be treated just like live line power from a regulatory point of view. Think about it. If it's not isolated, it is live line power.