Yes, as if it were the black key. My thought would become more clear, if instead of the rubber you place a book (use the spine of it where your nails should be facing), just to use the different level (about 1-2 cm) of two surfaces (The rubber may be moving). Then you can make up three levels, so the knuckles are exersised, without strain though. Then look for something round (a taw, a cylinder shaped battery) and use it for rolling your finger. These are exercises for back and forth movement of the fingers. Use something similar for right and left movements (that would be perhaps by placing the rubber on the surface, put one finger on it, leave some space at sides of the rubber (fingers open). So you go: the finger on the rubber, then down at your left, up the rubber, down at your right (sidelong movements). Don't push your fingers on the surfaces, don't injure them. There is no need for pushing, it's like holding a key down. That requires the minimum strenght.
These are simple exercises, if you put them down they will be done by themselves, I mean that you don't have to try that hard. Then someone may use the same table, lay their fore-arm as before but letting the hand jerking out of the table, feel your carpus loose. Make circular movements, up and down... Be inventive and inspired!
I hope I gave you an idea. Make it applicable for your needs. Let me know if that helped. (After such gymnastics I have a different feel when playing scales or even trills.)