The Mustang types came out around the time I was retiring, so I can't give you personal, hands-on experience.
However, I've looked at several schematics, and it does appear that the trigger pin is just a straight pin with no retaining feature hidden inside.
Pins, particularly straight pins in aluminum frames have to be installed fairly tight, or they'll vibrate out under recoil.
Here's how to remove a resistant pin:
First, buy or make a "starter" pin punch.
This is nothing more than a standard punch that's been cut off to a working length of about 1/4" inch. This starter punch's short length prevents the punch from flexing and bending, dissipating the force.
The starter punch directs all it's force to the pin, and is used to start out resistant pins, which once you've got moving, you use a standard punch.
Next, get the frame in a well braced, no-bounce setup.
If the frame can move, shift, or bounce, the force of the blow is dissipated, and the pin may actually deform, locking it even tighter in place.
Make sure the pin has room to move out of the frame, and won't contact the bench, preventing it from moving out.
If needed, have a buddy position and hold the frame steady.
Put a squirt of Kroil penetrating fluid on the pin, both sides, and inside the frame, and allow this to soak over night.
Start the pin out from the LEFT side of the frame, using the starter punch, and a small hammer, using firm blows, and keeping the punch on the pin so the frame isn't scarred.
If this either doesn't work, OR you're just not sure you want to do this, I'd recommend sending the gun in to Colt, or Pittsburgh Handgun Headquarters and let them do it.
Better a few dollars in shipping than ruin a hard to find gun.