I just made a post about vests, but I had another idea... Are there any pants with a pocket large enough for a full-size handgun that would function similar to a drop-leg holster?

Walmart kahkis with cargo pockets. Some 5.11 pants have weird pockets like that.

Any brand of tac pants with cargo pockets that have elastic for ar mags.

Just get a jmcustom AIWB holster

OG107 OD Jungle fatigues used to have a smaller button flapped pocket in left cargo pocket for the CEOI book.  However, a J frame 442 or a Beretta 21A would fit in there perfectly.  442 is almost the size of a G19.  Didn't have one back then to compare.  The outside cargo pocket bellowed out would mask the pistol and since the gun was attached to the inside of the trousers, pocket didn't sag from weight.  

You could just have sew in a cheap nylon holster to inside of cargo pocket.  Front pockets of BDUs were also large enough for a full size Beretta M9 and still not stick out.

CD

1) What's the point of a drop-leg holster if you're not wearing armor?  And if you're wearing armor, do you also need to conceal the gun?

2) Carry methods that rely on a specific feature of a specific piece of clothing are generally not the best idea IMO.