The blocks are good old fashioned, pain-in-the-ass BSP. Around 1700 brushes in fact. It's deceptively simple because of the blocky textures and shapes - it's a lot easier to notice when things look wrong than when they look right. My goal was to make it look as accurate as possible to Minecraft without impacting gameplay adversely.
I tested importing geometry directly from Minecraft at the beginning of the project, but even small sections were too complex and buggy to use. In retrospect, I could have modeled large chunks and structures as meshes, but meshes have plenty of drawbacks. Collision, culling, optimization and lighting all would have been issues if I worked mostly with meshes. Lighting is especially tricky when you try to blend BSP and meshes together that are supposed to be the same materials. It's possible with some tricks like darker textures and special lit light sources (like I used on the hidden doors,) but they don't look match up very well without some effort.
Minecraft uses a 16x16 texture for a typical block, which looks terrible when imported to UT2004. I upscaled them to at least 64x64 and set them to LODSET_Interface. I found out about a couple of other maps after I started this project (AS-MinecraftRace and VCTF-MCMountain) so I checked out their approaches to textures. In Minecraft Race the author used uncompressed 128x128 textures which greatly bloated the map size. MC Mountain's textures were also at 64x64, but at lodset_world - since he only used 64x64 blocks he didn't have much of a blurring problem, but he did have some issues on the plants. In my map I made several mix textures to simulate the "natural" block patterns in Minecraft, these do noticeably blur on LODSET_World. While the blurring is a function for optimizing, it had an adverse effect at "normal" texture settings when I tested it. Compared to a typical DM map with a lot of static meshes, I didn't have much texture load in the level and considerably smaller textures for the most part, so didn't think it would be an issue and used LODSET_Interface to keep them crisp-looking.
Regarding, your question, I have actually done something like that for the Mario Kart Wii community, by
making my meshes available for use, and even
curating a collection of models from other users. And they do get used in a lot of newbie levels.
In this case, I made a number of custom textures, models, pickups and emitters. I certainly don't mind if someone wants to use the assets from the final version of map for another Minecraft level, but they won't get results like mine without a lot of work and know-how. I'll be happy to give advice or help as needed. They will need to credit me, and of course Mojang.
Before any of that happens, I want to get this version tested and bug free. I don't know what you guys typically do in your map edits, so I'd appreciate it if you'd look it over, test it online, and let me know what it might need here.
The only other thing I can think of that might be cause issues are the textures being set to LODSET_interface so that they don't blur, but the overall texture load on this map is less than the average DM map.