Rather than mixed BASIC and assembly, I decided to leverage a few snippets of code I wrote for DC2N3 (such as the menu system) for use in USB-Commander. This means I am now using the cc65 tool-chain, in particular its C compiler 🙂
As it stands, the best IO interface to use for this task is the “conio” one. I got up and running in a few minutes this morning and have the menu system alive and kicking. I still need to figure out the definitions for cursors and special keys of the C64 (such as the left arrow), but I am nearly at full speed here.