I'll try and do a tutorial or preparing units for conversion in the next 24 hours, but while your waiting here's what I've found. Probably only meaningful to those who understand Blender already.
The key to porting units successfully is to combine all the separate meshes into a single mesh with a single material and texture. A mesh can contain disjoint parts that so there is no problem with horse riders, etc.
I decided to take the
mounted Conquistador I made for Civ 4 as an example of a pretty complex multi mesh / multi texture custom unit. The good news is the process works.
In this example I have to combine 4 256x256 textures into a single 512x512 one and then rescale and position the UV mappings. In the UV view S (for Scale) followed .5 will half the scale, S, X, .5 will half the scale in the X axis if you only need to combine two square textures into a single rectangular one.
My combined texture with fixed UV mapping in Blender:
Next.
1. Combine all meshes together using Ctrl-J.
2. Delete all materials but one - set the whole combined mesh to use that one material.
3. Rig any weapons to an appropriate bone e.g. BIP R Hand for swords, BIP L Forearm for shields. You can test export a NIF to check you don't have any unrigged (unweighted) vertices. If you have Blender will helpfully select them for you.
Now, test export as NIF and view in Nifskope. The whole unit should be a single Mesh (TriShape) with the newly made single texture. Also a good idea to test some animations at this point.
Now we export everything to FBX from Blender and use NexusBuddy to convert to GR2. Check in GrannyViewer - it shows a nice clean and complete mesh.
Note my model has 36 bones so I don't know how I've got away with breaking the 32 bone limit. The model looks dodgy in the Nexus 3D viewer but fine in game.
I'm pleased this conversion process seems to stand up to complex custom horseback units like this.