I generally use McKee's 37 APC followed by a light brushing where needed and a good flow of water. I generally don't apply anything further to it as it's just the undercarriage and so long as it's clean it looks good.
Clean as mentioned above. A powerwasher prewash is almost a necessity for severe dirt and mud.
Mothers Back to Black aerosol: Never used on fabric wheel liners, but have used on very tired/worn out looking plastic liners. B2B has a great darkening effect on the plastic. Almost like a black temporary paint. The aerosol would really be great for getting into the texture of the fabric.
Only downside to B2B aerosol is the horrible chemical smell. I used the product outside and I could smell the B2B for a very long time after using.