I've succeeded in adding new characters to the game! Since I wasted so much time at this, hopefully my pain can help someone else.
Writhing Play can animate any "Miku Miku Dance" character model in the .mmd or .pmx format
Software needed:
Unity version 5.5.6 (you can get it from the official Unity site).
MMD4Mecanim_Beta (from stereoarts.jp; Unity mod to import MMD/PMX files)
wpmodTools_170625.unitypackage (included with Writhing Play)
PmxEditor version 0.2.5.4 (Optional, for modifying character models)
Setup:
1. Install Unity. Just the base program is required.
2. Extract MMD4Mecanim_Beta to any directory.
3. Find the wpmodTools_170625.unitypackage in the game directory and note its location.
4. Run Unity. Create a new project. REMEMBER DIRECTORY. Default 3d.
5. Select Menu "Assets -> Import Package -> Custom Package". Select the wpmodTools from above and open, then import. Success will give you a "Writhing" menu at the top of Unity. The pulldown is in Japanese, but don't panic....
6. Again, Select Menu "Assets -> Import Package -> Custom Package". Select MMD4Mecanim.unitPackage from above and open, then import.
Character Model conversion
1. Go to your project directory. Go into the "Assets" directory. Create a subdirectory for your new character and copy your character files here.
2. Run Unity (if not already started). It will see the files and update.
3. Bottom Left Unity window, select your character directory under Assets. Select the character model file. The correct file has the Unity icon (3d box).
4. Once selected, MMD4Mecanim will show the readme and 3 checkboxes on the right side window. Check the three boxes and agree.
5. Same window now changes to show PMX/PMD file, VMD, and FBX Path, and a Process button. Press Process to import with MMD4Mecanim.
6. Once done, you should see a new Asset icon showing your 3d character (.fbx file). Drag it onto the scene in the middle window.
7. In the left window, the character should already be selected (highlighted).
8. In the right window, find "Transform". Change "Position" X, Y, and Z values to 0. You'll see the character moved in the center window.
9. Select Menu "Writhing -> <Last Option>". This is "Automatic Model Settings". You should see items added on the right window.
10. Select Menu "Writhing -> <Second Last Option>". This is export. The options here are to create the character file described in my previous post.
11. Edit the Character Name, model file name, voice file, male and female bits. All of this can also be edited later in the _characterInformation file.
12. Press the bottom button on the dialog to export. When finished, it will pop up the "Robi\Output" directory.
13. There should be a _characterInformation.txt file and a model_name_normal file.
14. This is it, copy those two files into the writhing play data "StreamingAssets" directory (VR and/or normal).
15. Run the game; choose freeplay mode. New character should be selectable.
Notes:
Don't save the unity project. You can reuse the same project for all the models you download.
If you create a new project, you'll have to import the packages again.
I haven't tried making voice files. It doesn't look difficult, but it would be tedious. So I just recycle the existing voices with new models.
Setting the Transform position to 0,0,0 is really important. Unless you want a character model 'glob' in the game.
Many wrong software versions were tried. Don't make my mistake. Don't attempt to use the software program "Blender".
I used PmxEditor to delete parts of character models that didn't work well in game. Describing this would require a separate post.
