The only issue I would have if The Smiler was enclosed would be what would it look like from the outside? Surely this would cause a bit on an eyesore? Having said that, I'd imagine the things that could be done on the inside could create a truly phenomenal experience, perhaps at a Disney level, if done well...
I think it would be really cool and would add some difference to the rest of the X-Sector if the outside was themed and looked like the large building ran by the Ministry of Joy, in the trailers leading up to The Smiler, you know? That building, with the Advocates dancing in a circle outside, or maybe, some sort of bunker or something themed to a Ministry of Joy testing area.
I know this would never happen, but if that were to happen, imagine the que being closed of from the ride as well, to make it look like your in a huge normal building. Then, when you enter the strobe room and the station, which has a distinct aesthetic to the rest of the building, you'd be creeped out as you realise the whole building actually incases a huge beast of a ride. The theming and objects on the rides could be lit up, now that would be a spectacular dark ride, beyond Disney level. Just imagine!