Tweeti have never carries with anything other than an IWB holster or a pocket holster for little peewee.
i can see either of those working great during the winter when i wear a hoodie most of the time. biggest pistol i carry is my XD9 sub compact. never carry the fullsized pistols.
a glock 20 is a hand cannon. i had a 2nd gen 20 and sold it because it was so rowdy and just too damn big to carry (wish i still had it now)
i know guys who carry full sized 1911s in rigs like that. a 1911 is about the same length but the glock is chunkier.
if you decide to do it let us know how it works out.