i have added to my bash rc the exact text you posted on this link
http://materm.sourceforge.net/wiki/Main/Documentation?action=download&upname=faq.txt
and also the one on this link
http://materm.sourceforge.net/wiki/FAQ/Tips
it changes the tab title, but the text there is the echo comand and some text is outputed to the teminal(see attach)
im using fedora core 4, with fluxbox 1.0rc2 (compiled manually)
screen capture
Logged In: YES
user_id=1493669
Originator: YES
sorry, i forgot to say that this happens with and without the "Mrxvt.syncTabTitle: true" option in the file ~/.mrxvt