HGSS trees use multiple polygons, one for each different texture. One texture is the root, another is the main body of the tree and the last one is the side. Developers probably forgot to add the last one to that tree, so that's why you see it like that.
You said maps didn't use tiles, so that should mean each tree is its own 3D model, right? If so, how is part of it missing? (I think it might be because there are two types of trees -- ones with the little sides, and ones without.) Kinda curious, though.
I did a tiny search, almost not really a search, but the flag for beating the Hall of Fame is 2434. If the programming is neat, it should be one of the final flags. I thought, hopefully.
But, unfortunately, legendary Pokemon seem to use random flags. Heatran uses 2435, the flag right after the Hall of Fame, yet Shaymin uses 2517 and Darkrai and Cresselia use 606 and 621.
Anyway, the point is, I've never seen any 2600+ flags used, however, they could also be unstable, if they even exist. I can't test, so if you want to make a backup and try go for it.
By the way, the Old Chateau uses flag 666 when dealing the with the ghost girl. Gamefreak is silly
I do many edits in the Diamond scripts, but I also keep the original ones in most parts of the game because the hack is an improved Diamond version, not a brand-new game. Thanks for offering for help, if there's something I need help with I'll tell you.
For now I only need to find 10 free flags (not used by any script). Do you think you can do it?