I have a pre -decocker  pt92.

These have the same screw spacing for grips as the beretta.

What are the chances I can replace my safety with a beretta hammer pin and cover everything up with beretta grips resulting in a functional gun with no safeyy

I did some extensive work to my PT 92  including relocating the screw bushing so it would accept modified Beretta grips. to answer your question, yes the Beretta hammer pin does fit as does the Beretta grip, however unless your brave enough to tackle milling the groove in the slide or get a 92 F smaller head  hammer pin.  I took a few pics for ya... hope this helps...

So if I just get the small head pin, it will work just fine without modification?

without actually trying it myself I cannot answer that... in theory it should. Keep in mind the larger pin head that rides in the slide was designed as a safety to prevent the slide from being blown off the back of the frame...  people still use the early SB and F models that have the small head pin and have no issues.  I had a 92F compact and fired thousands of rounds with no failure.
maybe I'll try this on my next build if I can find another donor PT... they are getting scarce around here.

The stock safety that will be replaced by the pin doesn't capture the slide to the frame now... so I fail to see any difference

Well it worked!

Took about 10 min start to finish.

I used the new pin to chase the safety out with a couple light hammer taps without even removing the hammer spring.