For those who don't know, there is a type of low-heeled ladies' shoe called "pumps".
Start with those inside liners for making shoes more comfortable. They normally are filled with a gel, but I think custard will work about as well.
Next, the heel of the shoe has a segmented hollow/telescoping
design, like an old-fashioned car antenna, only thicker, of course. The part of the heel that contacts the ground needs to be very durable, yet replace-able when needed.
We fill that hollow heel with custard, too. We connect the insole/liner to the heel, so that the custard in the insole liner can act as a reservoir. A small mechanical ordinary-type-of-pump and valve system lets us pump custard from the insole liner into the heel, or vice-versa. The valve is a simple open/close thing, opened to let us pump custard one way or the other, and closed to ensure that the amount of custard in each part of the shoe stays there, after it has been pumped.
As custard goes into the heel, it telescopes toward maximum length (back of shoe gets higher, relative to the ground). The custard does its solidifying-under-shock thing, and supports the load when someone is walking in this shoe.
When a more comfortable arrangement is desired, the small valve is opened and the wearer's weight compresses the heels, forcing custard back into the sole-liner. The thickened custard-filled liner is more comfortable because when a foot impacts it, it is a fleshy impact that is spread over the whole sole of the foot, probably not enough to make the custard solidify).