On your compatibility list I could't find any notice about the possibility to use Weeder digital I/O boards form Weeder Technologies with Pluto.
Those boards are plugged into serial ports and can drive up to 14 digital logic I/O channel. Moreover they can be cascaded.
Please have a look on their web site for more references (http://www.weedtech.com
They can be used for a wide variety of purpouses (drive relays and triacs, decode small keypad, and more) and last but not least they are cheap.
Is there any existing template already available to use them with Pluto?