I've never tried to disable a factory ATA, at least on a networked car, but here's what I would try. Dis-connecting the alarm horn is simple enough, but you've still got to stop the lights from flashing. There's a signal wire between the PSE pump and the headlight switch which goes to 12V when the alarm is triggered. Try un-pinning this wire (PSE conn.4 -socket 1-YE/GN) and trip the alarm. Do this with the alarm horn still connected. If the horn sounds, but the lights don't flash, simply dis-connect the horn and you're done.

Make sure you check all other PSE functions when you're done. There shouldn't be any problem with RF locking/unlocking, but you'll obviously lose the "PANIC" feature.