At this point in time there is no support for the cart filesystem yet. I have
a lynx-cart-demo example project that uses an interrupt driven display,
has support for the cart filesystem and an abcmusic sound module.
At some point in time we may find a way to rewrite these to fit the way the
cc65 drivers require. But for the time being you can create less portable
applications using these Lynx specific modules in lynx-cart-demo.