Support for this is on the long-term roadmap, but we have no specific timeframe.
I'm wondering if you could extend the WMS provider to handle the time parameter. From there you could define a callback method on the clock.tick to decide if the time variable on your custom wms provider should be updated.
You'll want to be careful on how often you're updating that variable and I'm not sure if you'll have to call a refresh imagery to have Cesium detect it needs to refresh the layer's tiles.
Might not be too bad, but I have not done it so my approach could be horribly off base. Kevin Ring, who wrote the original WMS provider might be able to provide some additional advice as well.