Well i stand corrected, not that I've ever really looked specifically on other boats but both my Adams40 and previously a Dunk 35' had the topping lift turning sheave on the top of mast not fixed at top.
0ne advantage of the t lift on the mast type is it can replace the main halyard should you lose the primary one, all other potential halyards awkwardly face forwards. So not really swayed either way - a bit like the anchor
Getting off topic now but any normal halyard load doubles its load as compression on mast as both the up and down portions are under same tension . A Halyard lock or stopper system only has the down tension with sail in use and usually lighter load when hauling. Admittedly halyards have much less tension than standing rigging
so perhaps of limited benefit anyway and certainly not for a retrofit.