We used to build rise time accelerators, but a quick patent search shows that faroudja and Crystal Vision have both used a similar process in some of their standalone comb filters. Anyone with a videophile mate got a Faroudja VS50 or Crystal Vision VPS-1?
They probably turn up on ebay or similar these days as they are not much use in the digital age to most people, and would probably be cheap. Keep yer eyes out for one.