Well I'll throw you some source that will do the trick in a bit....
how the text is displayed is basically down to the the controlling microchips/firmware on these LCD's. They tend to have a RS232 interface built in so the hard part is already done.
The fonts/scrolling can be setup in commands at the start of each line. for example when generating the text to send, if its greater than 24 characters (on a 24 character width LCD) then you can simply insert a scroll command at the start of that line, and that line only will scroll. However, as this changes depending on brand of LCD, I suggest that mamewah does not deal with that.
...basically all that can be setup beforehand in a txt file for each game - so all mamewah would have to do is open the txt file, open and setup a port (e.g. com1, 19200baud, etc..) and send the file to it. (and perhaps some error handling, such as revert to a default txt file). There is no fancy comms checking as it all the comms can be one way.
This method also means that people can customise the txt files to work with their lcd and match their own control setup.
I'm having a go now at knocking together an app in VB similar to the one maraxle had for generating the txt files before he took it down. (unfortunately parsing xml files in VB6 is new to me... anytips?)