Is it possible to *proportionally* reduce a DC voltage supply?
An open-loop buck converter could be used, preferably with synchronous rectification. 

The switches should operate with a 21% duty cycle, which will give you about 0-5V. There will be some droop with increasing current, due to the inductor resistance (typically a large contributor) and Rds(on) of the two MOSFETs. 



Edit: Conceptual diagram:-



S1 and S2 can be MOSFETs or S2 could be a Schottky diode. S1 closes with duty cycle D and S2 closes for D' = 1-D where D ~= 0.21 . The period Ts is kept short enough that there is only maybe 15% current ripple in L and C is chosen to keep the output ripple voltage within requirements.

For example, if the frequency is 100kHz then Ts = 10usec and S1 would close for 2.1usec and then open, and S2 would close for 7.9usec and open, and repeat forever.  

Note that if S1 and S2 are closed at the same time, even for a relatively short time, bad things will happen as they would be shorting the supply out. 



To get better accuracy would require a closed-loop control circuit- measuring the '24V' supply and servoing the output to a fixed fraction of it. That would be a bit more complex.