Assuming the C-64 has no terminal software available, my best bet appears to be to use Star Commander on a PC running MS-DOS. I'll connect the PC to the 1541 disk drive using a special cable (probably the XA1541). I'll use Star Commander to write Commodore software like EBBS to a floppy in the 1541. Once the software is available to the C-64, I can try setting up a BBS like that run by Leif Bloomquist. Using these instructions, I could even access telnet services from the C-64! Here's another option called BBSLink that forwards incoming telnet connections to a Windows box to the COM port, where the C-64 listens. Maybe I'll connect the C-64 to my packet radio TNC, and really go retro.
I also found these how-to's to be helpful in figuring out this process, and an incredible amount of original documentation is online. Because Star Commander works only in a true MS-DOS environment, I started looking at what could be done in DOS. Would you believe people are running DOS-based web servers?