tn5250.sourceforge.net has a lib5250 and there's also a 3270 emulator available as x3270.bgp.nu (BSD license). Maybe it is possible to integrate support for these telnet extensions documented by IBM in a bunch of RFCs into Termius? So sysadmins of Mainframes and AS/400 (or IBM i as it is called today) can do their duty in one convenient place.