JH> Is anyone aware of any software / driver for Linux that will, like
 JH> NetSerial for example, emulate a "modem" over TCP/IP. That is, I can send
 JH> "ATDT123.123.123.123" and it will establish a telnet session with the
 JH> specified IP address and "act" like a modem?

Not software, and you're probably already aware of it, but there's the
WiFi232. It's designed to attach to old devices and acts like a modem but
routes the connection over TCP/IP. It accepts ATDT127.0.0.1 and functions
exactly like you're looking.

--- Mystic BBS v1.12 A36 2017/11/07 (Windows/32)