An 800? No idea. If IRM doesn't do it, none would. Would it sell? Absolutely!
Now whether it would sell enough to make it worthwhile versus say an A class with 17 variants, I doubt that. I know that collectors would get one or multiples or three if 801 and 802 were available. Personally I would run mine especially if we had some 1950s laminate stock to go with it.
The price point cannot be higher than paying nearly STG300 for a OOworks J15 or attempting to assemble an 800 in brass. Over the years, we have seen some fine examples by experienced brass builders and some not so fine ones by those attempting an 800 but unable to complete it to a satisfactory standard as a complicated build. One could commission your build but that would makes a pretty high price point also.
Maybe you could sell it with some laminates as the newer Bachmann 00651 set which was produced both for the European market and also the US market. That set is still sought after even thought the loco is not entirely prototypical (but close) and the carriages are not prototypical.
I have no doubt Park Royals will be produced at some point but whether I'll be around to see it I don't know
Anyway to get back to topic, I bought a weed sprayer set for the flats and planned to repurpose the container and tanks. It arrived, the wife saw it and said it was a loverly little thing when I explained what it was for, so now I'lll have to look for some headboards with wasp stripes for one of my 141s. It can go on display for now
I guess we'll need some 20' CIE tanks then too