You don't really have "TCP/IP to serial bridges". TCP & IP are network protocols, serial is a physical layer thing. Eg you could have ethernet to serial, or wireless to serial.
PPP and SLIP are the two main ways of encapsulating IP packets on a serial link.
You can happily plug a compact mac with MacPPP into a Windows box running RAS or UNIX server running PPPD, only cost is a normal mac serial cable, but you are limited to RS/232 speeds.
Using LocalTalk lets the Mac talk 230kbps.
If you want a serial link out of a Mac that does work at 230kbps, you already have one, it's called 'ADSP Tool'. So all you would need is some component on the network that can read/write ADSP....
Did I mention my "adspd" daemon? It works rather like "inetd" but for ADSP.
So in theory you could make a shim serial port (see Inside Communications Toolbox page 182) that registers itself with CTB, lets the MacPPP use it as a real serial port, then talks ADSP out to a server which then just talks directly to pppd. No major technical problem there, except that MacIP (IP over AppleTalk) already does this in a much better manner.