Unless you used the car routinely, a hackier solution would be just to buy out a block of rooms in the sleeper car.

Amtrak.com suggests rooms from Seattle to LA are ~$700 each. https://www.craigmashburn.com/amtrakcardiagrams.html says a Superliner sleeper car has 20 rooms. So you could rent out an entire car for about $14,000. Complementary meals included!