I have a version of Palm Threaded SMS that I personally removed all the MMS related stuff from it as far as how it grabs the MMS when they arrive. Works good as far as MMS goes. All of that goes to Arcsoft by default like it's supposed to. But, again, personally I like the new Extreme Text that is out now, and it works along side of the normal SMS so it should not have any side affects.
Link to Extreme Text:
http://www.birdsoft.biz/extremetext.htm
Not sure I can post the Palm CAB, but I'll put it on an FTP somewhere, and post link later if I can. But please, try Extreme Text. I like it.