Rack onto yeast cake then fast rack to secondary?

Don't wash with wort, you won't get the chance. It will take off before what you want to extract settles. Just rack on top the cake and use it for what it is.

As long as you practice good sanitation to the fermentor while racking the old beer out and the new in you can keep it longer. Use a sanitary siphon style cane, or wrap the cane and access port with foil.

I've kept a yeast cake in the fermentor for over a week and pitched on top of it. Just keep is sealed and cool. If you can refridgerate to 38 or so it should keep as long as new yeast.

Couple notes on racking on top of a primary cake.

1. Be mindful of the contence of the last beer. Dry hops etc can still impart flavors into the new beer.

2. Doing this usually results in an over pitch of yeast. Which is fine for most clean fermentation styles, but not recomended if the style calls for yeast growth esters as they will be very minimal or none at all. For example, I've gotten almost clean lager quality from London ale yeast which is usually fruity.

3. Be prepared for a mess. Use a blow off tube etc. When pitching on a healthy cake it usually takes off very vigorous in just a couple hours.

4. I've only done this twice back to back, some say you can go 4-5 times then the yeast kinda poops out. Because your basically using the same yeast cells and new growth is minimal.

5. If the yeast fed on too much simple sugars the first batch, it may not have the pathways for more complex sugars anymore and may give up on the second batch.

Hope this helps.

It's probably as safe as anything in a sanitary environment, though if I'm understanding you correctly it means another, however small, potential window of exposure to dangerous microbes since you're racking twice. I've reused yeast a few times to no ill effect, but a lot of literature advises against doing it more than that.

It also sounds like more work to me? Why not just rack some of your slurry into a sanitary container immediately after you rack the prior batch? You could even keep some sanitized wart in the fridge, say in 2L soda bottles, for this purpose. Assuming you brew immediately after that racking, that's at least an hour or two to let the liquid you want to decant settle.