@tpr not in mame2003, however mame2003-plus just added official support for the middle mouse click. 😎😃 You can update or install it from source to get the new input.

I confirm the same issue. Also, controls appear unasigned under the retroarch quick menu option, unlike other mame versions (2003, 2010). The only way to remap controls is via the tab menu, but secondary mouse buttons do not work