You can use FileSystemObject to read the file then set the text to Messenger.MyPersonalMessage
js code:
try { Messenger.MyPersonalMessage = GetEverything('c:\\test.log'); } catch (e) {}
function GetEverything(sFile) {
var fso;
fso = new ActiveXObject("Scripting.FileSystemObject");
return fso.OpenTextFile(sFile, 1 /* ForReading */).ReadAll();
}