If you wanted to fake some of what a Mac Charlie was capable of today you could connect a compact Mac to a Linux (Or other x86 *nix) machine via a serial cable, get a console via a terminal program, and run DOSEMU. The end result will be a (very warty) version of Mac Charlie-esque PC/Macintosh integration so far as running software goes. (You'll get copy and paste to/from DOS programs via the terminal emulator, which is basically all the Mac Charlie did.)

Of course, if you're going all-out and want a new "hardware" Mac Charlie another option would be to lay hands on a PC Weasel "video card". The MDA version is essentially what the Mac Charlie had built into it.

I suppose that would be an interesting yet demented project to write a custom terminal program for Classic macs that could drive a connected DOSEMU session or a PC Weasel as "seamlessly" as the Mac Charlie application did its custom hardware. Of course, unless you want to inflict horrible keyboard pain on the user you'd have to limit it to running on SE's or newer with Extended keyboards. You're not likely to find a source for MacCharlie keyboard extenders. :^b

(Maybe you could do a dropdown onscreen keyboard to facilitate generating keystrokes for the missing keys?)