You are right that plucking near the neck produces a nice tone - many people play slap - also many people like the ability to vary their plucking intensity as well to vary tone. The thumb rest, as indeed the tug bar, are generally items applied to Fender basses in the mistaken belief that everyone plays the bass in the same style - there are lots of reasons why the thumb rest shouldn't be there but the main one is restricting playing style. Possibly a design fault, incorporated for the best of reasons. That said, if I had a vintage Fender I would want all the proper bits on it, whether they're ill conceived nonsense or not.
I'm not sure anyone has made it clear that it needs to be a bridge ashtray - why should it be - it is intended purely as an aesthetic addition which will limit plucking area and at the bridge, prevent access to the mute system on many Stingrays. There are also dual pick up Stingrays, but as this ash tray will serve no useful purpose why not put a neck pick up one a la Marcus Miller on a bass with no neck pick up - it would make an aesthetic difference, but as with a Fender, create little practical benefit. Before people start drilling their Stingrays (notwithstanding the mojo mentioned earlier) a photoshop mock up would be a better idea as it may look awful as well as the other disadvantages mentioned.
The Fender thumbrest (in the '70s position) doesn't get in the way of slap for me, any more than the lower strings themselves do when I'm slapping a higher string. I find it actually helps me make sure that I slap the E the same as the others without making a bigger arc with my thumb. I don't think it's installed in a mistaken belief that everyone plays the same way - on the contrary, it gives me an extra spot to put my thumb and I miss it when I play basses that don't have one.
As for functionality...well, not everything on our instruments is functional - do you play mostly fingerstyle, yet keep a pickguard on your bass? Bust some players developed styles that relied on the pickup cover being there - Marcus Miller has talked about using the cover as part of his slap technique, and James Jamerson famously rested his hand on the cover and plucked with one finger.
Actually, speaking of P-bass pickup covers, I wonder if one of those would fit over the Stingray pickup? It's probably a similar width.