Typical example of "YMMV":
Armour damage on a crit can be reasoned, but keep in mind that this gives those fighters an extra advantage who specialize in criticals. I.e. Barbarians (Versatility) and anyone who takes Improved/Greater critical.
A combatant with I/G Crit and a suitable weapon (particulary scimitar or the like) will score threats almost half the time. Even if he has bad damage (due to low strength) he can utterly destroy even Full Plate armour within just a few rounds.
I'd be very careful with this. Also, it isn't coherent with the system since the idea is that heavy piercing weapons are best at penetrating armour, while curved blades are better against light or no armour.
Compare:
Scimitar, crit 18-20/x2, AP2
Warhammer, crit x3, AP7
So in short, if you want armour to be damaged more easily in combat, you should devise a mechanism that has to do with AP rating rather than crit rating.
Then again, the RAW does already indirectly incorporate the AP rating in armour damage, since high AP means that half the DR is ignored, so the target takes more damage, which makes the magical 20 easier to attain.
(You could of course say the same about criticals, of course.)
I have a feeling that the main intention of the current armour damage rule is to keep PCs from salvaging armour from foes killed by massive damage. After all, who'd want a Breastplate with DR3? Right.
With a house rule to faciliate damaging armour, you encourage a whole new combat tactics. Not that this has to be bad, but be aware that it may turn out as a rather drastic change in practice.