Thanks a lot Osso,
for the moment, I solved creating 2 different launching script (I'm using SDLMAME):
one that contains: ./mame64 -nouse_bezels
an another with: ./mame64 -use_bezels
so I can use the fist one when I'm on the laptop, and the second one when the external display is connected.
This could be a good solution (it works over all the games, so it's perfect),
but I still think adding the feature to toggle between the artwork options, like you can do with slider controls, could be a good improvement.
An even better solution may be always keeping bezels in use and instead:
a script that contains: ./mame64 -artwork_crop
and another with: ./mame64 -noartwork_crop
With "-artwork_crop", priority is given first to maximize screen space for the game itself to fill in properly, then add artwork accordingly. Typically, on a widescreen display, the top and bottom of the bezel is cropped, while part of the bezel's sides will still be present.
Utilizing "-noartwork_crop", priority is given first to display fully whatever artwork options are turned on and available (I.E. Entire bezel will show), then fill the game into the remaining area.