Based off of the post by u/Mirror2and2 Now you can turn Sheep into blocks! (Assuming you've installed Better Than Wolves 2.93 or 2.94 and Modloader)
Unlike those companion cubes you get from wolves, Sheep blocks are extra durable and cannot be broken in the millstone, nor can they be cut by saws! As a matter of fact, they'll break any saw you put them next to!
Link: https://www.mediafire.com/file/m0d0kostduej6nq/BTW+Sheep+Block+Addon.zip/file
Modded blocks can be added to MCEdit's registry by editing /library (must be unzipped)/pymclevel/minecraft.yaml. You must re-zip the contents of the library zip file in order for your changes to have an effect.
Be sure to read the instructions within the .yaml file. The most useful properties are brightness: (light level) and opacity:0, as these will eliminate worlds with modded blocks from having lighting issues after being edited in MCEdit. Be extremely careful with spaces, as this will cause McEdit to deliver a parsing error and no textures will load.
Textures can only work through adding them into the terrain.png. Using mlblocks: in the .yaml file doesn't seem to affect how MCEdit reads textures. Attached is an example of the terrain.png file I had to make for mod support. I think around ten textures are unused.
The mod support is limited in the fact that mod blocks will not render like vanilla blocks in MCEdit (see the picture with the detector rails, the stone one is vanilla, the other two are Better Than Wolves) and modded blocks cannot be rotated correctly when being moved or copied in MCEdit.
Currently, I don't know of a way to support for items. Editing library/pymclevel/items.txt doesn't seem to change anything
Also, you can make a terrain.png with texture pack textures to have mod support and a texture pack.
This can be done by replacing the terrain.png in McEdit's files. Just unzip the library zip file, replace terrain.png, then re-zip the library file. You may also be able to add mod support for modded block textures via changing a YAML file. This may cause a crash however.