I made a similar request (with more detail) in
Needed: Customization for ePub, which is for the EZ Reader (Hanlin) but is equally applicable to any implementation of mobile ADE
Some implementations (Sony, Jinke, Bookeen) already have a way to access user-provided fonts in ePubs, see for example
Fonts and Epub - What works on Sony, Works for Pocket Pro too!. This is a standard feature of mobile ADE, but the fonts need to be in a particular location (i.e. in a particular folder) - and this location varies between devices. This capability is required in addition to a user-provided userStyle.css in order to have user-selectable default fonts.