CUSTOM LOFT (KINDA)
AKA MODEL SWAPPING
You can copy other models from the game into the loft by inserting them into the "the_loft_plant.nif" model. You can't directly add them to or modify the loft.nif because it will kill the camera and animations.
You need the toolkit, nifskope and a program that can edit dds files.
The guide is using the plant.nif to add extra posters but you can (probably) add other models like from the guitarcade minigames. Or can also do this to add other models to other stuff as long as they don't have animations (.kf .kfm files)
Here's an already done one with extra posters (Paste contents in "assets\generic\env\the_loft"). You can just change the textures, paste it in and repack if you want. Or use the poster locations as reference for placing your own objects in-game since they're extra added in all main locations.
Guide
Make a copy of and unpack "venues\loft01.psarc"
Go to "assets\generic\env\the_loft" and replace "loft_sway_plant.dds" with a blank dds file. This is a workaround to make it invisible since deleting the mesh crashes the game.
Open "the_loft_plant.nif" and "the_loft_main.nif" in 2 seperate nifskope windows.
In the "the_loft_main.nif" click on the poster to select the "NiMesh" and then click on the "NiNode" it is parented to.
Right click on it and select block, copy branch
In the "the_loft_plant.nif" nifskope click on the "NiNode" scene root and then right click, block, paste branch.
Go to "assets\generic\env\the_loft" and make a copy of "loft_posters.dds". Give it a name with no capitals or spaces. Open it in an image editor, change the poster on the right to something else and replace the rest of the image with black. (then save it)
Now in the "the_loft_plant.nif" click on the poster and in the block list right click on "NiSourceTexture" and do texture, choose.
Select the texture you just made and in block details change the file name index from the full file path to just the file name.
eg
e:\steamlibrary\steamapps\common\rocksmith2014\venues\loft01_psarc_rs2014_pc\assets\generic\env\the_loft\tut.dds
to
tut.dds
By default its location is too far back to be seen, you need to increase it's X position or it will be through the wall.
Right click, transform, edit. Then when you are done, right click, transform, apply. Then save the nif.
I can't give much advice for the positions, it's just a lot of trial and error. You need to repack the loft.psarc and resave the nif each time and adjust accordingly.
I'll link my own plant.nif somewhere (check top), it's got posters added in all the main locations (guitarcade, main screen, las selection and an extra near the 3 posters on the wall) and you can use them for reference.
Also since this is through the plant model not the loft itself there is no real lighting/shadows so the model may be too dark. The easiest way to fix this is to change the emission value of the material property. If it's in a dark corner have a lower value so it looks darker.
0.4 for all values works well for a nice lit area and leave it on 0 for dark places.
That's about it for now.