I think that there are some European high-class cars with 6 valves per cylinder. Three-way cross-heads I assume? Five valves are pretty common...
24 valve heads flow much better than 12 valve heads mod-for-mod, correct? PDR says so. And P-pumped 24 valves seem to be getting awfully popular on the competitive end of things...
http://www.pdrdiesel.com/oldsite/DodgePortedHead.htm
I'd do it to get the better pistons, the better rods, the engine position sensors, the better lubrication and coolant systems, the better valve covers, the arguably better valve train, et cetera, all in one $2000 eBay package.
24 valve block, VE timing cover, injection pump gear, injection pump, probably will need custom lines, stock crossover tubes, custom injectors. I'd suspect that stock 2nd gen injector pop pressures could be dropped enough to work. I think that the only thing custom required would be injection lines?
I'm poor so it's probably be better to just rebuild a 12 valve long block with some more recent goodies inside and spend money on a modified 12 valve head. But I don't think it'd be that hard or foolish to put a VE pump on a 24 valve engine. Well, foolish within the context of somebody who has already crippled themselves with the VE pump for the sake of being different.
1990 D-250 Regular Cab: Tweaked injection pump, built transmission, a cataclysmic charlie foxtrot of electronics, the most intense street-ran water injection system in the country, and some more unique stuff.