Hey guys,
I know this thread is almost a year old now, but given there was never a final solution to this I thought I'd let you know what I've dsicovered:

I had exactly the same problem, RS232 to Aquavision works fine from my Laptop in Hyperterminal or docklight, but sending the same string from Control4 doesn't work at all. I can confirm the string is outputting correctly from Control4 by using Docklight aswell.

After hounding Aquavision for an answer we finally got them to suggested that a 200ms delay between each ASCII character will fix the problem. It seems the serial hardware in the TV has a very poor buffering capability, and so the string output from the control system was simply arriving too quickly for the TV to process. That's why when typing the characters on a keyboard it worked fine because of the inherent delay between characters when you type.

Hope this helps someone out there. I spent two days trying to solve this one.