Kermit 7.0 serves as a telnet client that will auto-download via Zmodem when initiated by the server/bbs.
Ztelnet, found at http://ibiblio.org/pub/Linux/system/network/telnet also supports that.
Also useful is Virtmodem found in http://ibiblio.org/pub/Linux/apps/serialcomm/dialout/ which comes with a script that calls minicom over telnet so that you have all the features of minicom.