So you have verified the pronto extender pin out vs. the meridian TX/rcv config? If so, and the port info etc is correct in the .xcf, it sure sounds like a control string issue. Otherwise, I am not sure where this could be going wrong. Other than typos in strings, pin error, and non-updated firmware in the extender, I have not encountered any RS232 issues in my system, with my dealers, or in the field so far.
Wish I could help. Philips has a new call center in the US for Pronto, 1-800-233-8413 (Option 1) or sent via email to prontosupport@philips.com, you might also give them a try. Keep us posted.
Jim