It is most likely unfinished features that were ripped out at the end of production or just something to make Mr. Backlot just seem more suspicious to players, which it does succeed in doing (otherwise this thread wouldn't be here).
My guess does go to unfinished features, though in past games, when they ripped out unfinished features, they tore the door away, not just left an NPC blocking it (IE: Safari Zone in G/S/C). I have yet to see any map data for Diamond/Pearl (has anyone gotten that far in taking it apart?), but I would bet we would find a few more pieces of Backlot's mansion, or remnants of events that may have been.
That statue behind him is quite odd as well, usually a police officer is guarding it, but when he's out patrolling you can mess with it. Might be a "fun" feature or possibly the beginning of more for Backlot. There isn't even anything really pointing to him post-National Dex, so maybe there was going to be an event to lead up to him and letting players know his garden changed and had some old Pokemon (since I haven't seen any indication of this feature other than online guides).
Erg, just me blabbing on and on. Just like that blasted Tootsie Roll Pop, we may never know the truth behind what Game Freak had planned (or didn't at all!) for their Backlot center of their Pokemon Pop. Cursed analogies.