It seems that newer version of PMX Editor have corrected the BUG (you know what does "bug" mean, do you?) that caused the malfunction when opening PMX files invalidated/corrupted by PMX Locker changing the fourth character in the file header definition. It seems I was right calling this supposed "locking" as "corrupting".
About PMX Model Locker: http://fav.me/db6nizr
What is PMX Model Locker?
It's a simple tool useful to "lock" a PMX file in order to prevent other users opening it and modifying it in PMX Editor. It preserves the original PMX model and makes a "locked" copy of it. The goal of this program is to allow MMD authors to redistribute only the "locked" version (and of course all its textures, spa/sph files, fx and so on), so final users still can use the model in MMD animations but they cannot edit it.
A "minor" issue of the "locked" model is that it won't work if used in