Well, I hate to make comment on your repair. I hope you have long term success.
The only reason I am replying is that I would hate to see someone make a mess of something doing this when the proper fix is so simple.
The problem you were warned about is that the end pin that goes into the band cup is limp and normally hangs at about a 45 deg angle to horizontal. It won't enter the cup unless it is made to stay straight. A little heavy soluable grease does the trick quite well. You can not see any of this once the piston is installed but there is no way to have it go together wrong.
Worse case if the plastic pin retainer comes off is that the valve body will have to be dropped to hold the pin as the assy is threaded in. If we are servicing the tranny at the same time we often just do this as its also a rather simple job. (Moaybe not a DIY though).
Bosch Master, ASE Master, L1
33 years MB technician