A couple of things to notice when exporting out of 3D Coat:
3D Coat: 1. After you select ‘Open in Original App’ make sure the boxes 1. Export Color 2. Export Specularity 3. Export Tangent Normal Map or Export Displacment are selected. By default these boxes are not checked.
If they are not checked then no maps will be exported.
In Houdini:
Go to the COAT_OUT node and elect the middle tab. If you changed the UV at all make sure you select Transfer UVs. I always select Transfer Normals if I use microvertex paint.
Make sure under textures the correct maps are selected. Then select ‘Get Maps Back’. If you Geometry comes in it may come back in larger, sometimes. You may also see the displacement on the mesh but no texture maps. This is fine.
Go to your object and dive into it. You will see a COAT_OUT and COAT_IN. Select COAT_IN and select the Textures Tab. Reload your ‘Textures’ from there. They will most likely be in you 3D Coat -> Exchange folder. To change this you have to set the path in COAT_OUT -> Output Folder to your textures folder. Your geometry and texture files will then be placed in the folder you designate. Not sure if 3D Coat will work effectively, I haven't tried. 3DCoat looks for the geo file in the Exchange folder.
Edit: When you reload your textures, load the textures with the .rat extension on the end.
A quick note: If your using PTEX in 3D Coat make sure you check the Use PTex box in COAT_IN -> Textures Tab -> under Normalmap.
Good Luck! Let me know if you need anything with it!