Unless you listen in the very near field (less than 3 to 4 feet away), you're probably not going to notice the slight offset of the tweeter / mid combo if the speaker is vertical. If the speaker is set horizontal, there might be a slight bit of image shift but it will lessen with distance.
Only way to know is to try it out. You can't lose money with vintage JBL's in good shape though.
I've been listening to them near-field and vertical for years. I've been listening to L112s vertically for decades. The 4412A will work as well as any 12-inch 3-way can in near-field. That's a lot of real estate to expect a cohesive sound stage up too close.
Okay, so here's my situation: I'm going to listen at extreme nearfield position (less than 2' from the speakers). JBL 120Ti's have very detailed, very precise imaging at that distance. But of course, the center image is ruined when you sit off-center, hence the need for center channel speaker. JBL 4412 is a very interesting choice to be the center channel speaker for JBL 120Ti mains, since they should sound the same (well at least their frequency response curves are very similar to each other). Moreover, 4412's are supposed to be oriented horizontally, so it would sit nicely above my computer monitor (my desk is against the wall).
However, how should the single 4412 be horizontally aligned? Should it be centered based on the baffle's vertical centerline? If that's the case, then the single loudspeaker would be symmetrically centered between the mains. However, the tweeter and squawker wouldn't. I wonder if such alignment would be detrimental to center image.
Or should it be centered according to the tweeter and squawker's position? If that's the case, then the tweeter and squawker would be perfectly centered between the mains, but the physical baffle would be slightly off-center, not to mention the woofer.
Since the 4412 is crossed at 800 Hz, then it seems a large portion of the midrange (vocal, etc) would be performed by the woofer. As such, it seems placement A (aligned by the baffle's centerline) would be better, but I don't have the chance to test it myself. I mean, the seller lives quite far away from my hometown.
Or maybe the 4412 should be oriented vertically anyway? But I read the model is designed to be oriented horizontally.