The FASTEST safety is one behind the triggerguard, especially if you use an oversize button, which is what many of the competition shooters use.
I disagree with this statement. In my experience, a safety mounted on the tang is way faster and more easily accessed than one mounted behind the trigger. Furthermore, imo, a safety mounted in
front of the trigger (ala, many, if not most Winchesters) is faster to use than one mounted behind the trigger. For instance, if you're coming up behind a point; ready for the bird to flush, with your index (shooting) finger extended forward, along side the receiver and directly above the trigger as it should be, I think it is a far more natural move to sweep your finger backwards to engage the forward-mounted safety
on your way to the trigger than it is to bring your finger
past the trigger to engage the rear-mounted safety and then have to return to the trigger in order to fire the gun.
For many shooters, what they've become used to trumps changing to a different method, no matter how more efficient another way might be. The fastest to access safety I've ever used is the one that Val Browning devised for his Double-Auto shotgun. For those unacquainted with this shotgun, the safety was mounted behind the trigger guard itself and required just moving the second finger, already situated on the grip immediately below the safety, a slight movement upwards to disengage it and fire the gun. The only potential downside to the Double-Auto's safety arrangement, imo, is that during the rigors associated with upland bird hunting, the safety can sometimes be
too easily disengaged. When hunting with
any shotgun, I always verify the status of the safety often during the course of a day's hunt but with the Double-Auto "safety check of the safety" was done even more often. And, yes, before someone chimes in with the old (but true) adage "the best safety is located between your ears", I agree that no mechanical safety should ever be substituted for the basic rules of gun safety.
As to the op's question, though I don't know of any gunsmith currently installing tang safeties on semi-auto shotguns, I do seem to remember an outfit regularly advertising in gun magazines concerning the installation of a tang safety on some Remington guns, either the 870 and/or 1100 shotguns and/or the 760 and/or 742 rifles. Unfortunately, I haven't seen said advertisement recently. I will do some checking on my reference material to see if I can find something of use.