The now-abandoned Psion "Series 3", "Revo", "Mako", "Series 5", "Series 7", "NetBook" and "MalayBook" organisers all have a decent LCD screen and keyboard, and their docking stations are designed to connect to a PC serial port. So if you AddASerialPort to your slug they can provide an interface to the slug via a null-modem cable, or you can connect one without any hardware hacking using a USB-Serial cable.
The easiest option is to download the Hermes terminal emulator software from http://www.iota.demon.co.uk/psion/hermes/hermes.html. This turns the Psion into a decent terminal emulator. Hermes is configurable up to quite large screen resolutions. On a Revo, 53x11 characters is nicely readable.
There is a known bug in the OS of the Revo and earlier Psion 5's which is documented in the Hermes help file, which causes the psion to lock up when software flow control is used. Either use hardware flow control (getty -h in inttab) or turn off flow control and set a slow baud rate. Hardware (RTS/CTS) flow control is better if you can get it to work. All Psion models except Revo come with a built-in Comms application which can also do file transfer (use Debian package "lrzsz" and use for example "sb -k somefile" to send "somefile" to the Psion by YModem-Batch - the -k makes it a little faster) and if you do that then you'll want a fast baud rate. However the built-in Comms doesn't keep up the display very well at fast rates, so it's better to use Hermes for normal terminal use and switch to Comms when you want to transfer files (or just use PPP see below, although that does use more memory on the Psion). Other things to note: hold down Fn when sending control characters; if setting the display area to less than the standard 80x24 then set
Hermes to NOT do auto-wrap and set the LINES and COLUMNS environment variables in your Linux shell; in Hermes press Fn-K to send a pipe character (or hold down control while typing its ASCII code 124); it may help to set incoming CR to CR+LF, Enter to generate CR and Backspace to generate Delete; if an application won't work then try running it inside "screen" (however other applications are better off when NOT in "screen") or choose an alternative. Hermes describes itself as vt100 compatible, but it's not 100% there, and the "screen" utility can send vt100 sequences that corrupt the display even when LINES and COLUMNS is set correctly. To work around this, set TERM to "ansi" not "vt100" (you can do this on the getty line in inittab). Note: Slight damage to the 3mx, Series 5 or Series 7 connector is likely to break the wires involved in RTS/CTS flow control. If no data can be sent when RTS/CTS flow control is selected, try using DSR/DTR flow control instead.
The Psions have a complete network stack that can connect to dialup ISPs via a serial modem. It should be possible to run pppd on the slug and have the psion connect to it, allowing use of the device's built-in email client and web browser. See also Palm