Never have.
Tried to go into my parts catalogs to see if I could figure out what the actual part number for that is.
No luck. Some of the older XJ parts catalogs show plugs.
But they really don't show you where they go. And most times don't show the specs.
And Googling the part numbers you do see wasn't really helpful either in finding specs.
So I poked around and found that.

 

I know this thread is a bit old but just came across it. The hole is where a control cable for an auto trans ,if you had one, went through the firewall. If the vehicle had a manual there was supposed to be a plug. About 1996 there was a TSB ( we got all TSB's where I worked, very useful) about the fact that the factory had forgotten the plug on some vehicles resulting in water and odors in the pass compartment (or in my case mice) and the repair was to install plug # xXxX. I have the TSB somewhere but..... In any case that's what the hole is

 

My '96 (AX15 gearbox) doesn't have that hole plugged at this time, if it ever has. Having it open hasn't given me any trouble