ok, that's a tough one.
is the 6bit input a additive or multiplicative value?
i. e. is the whole time delay = input * standard delay
or = input + standard delay

for single sparks you could use water.
1. bit: water channel with length "1"
2. bit: water channel with length "2"
3. bit: water channel with length "4" etc ...
due to the slow current speed in water, this would be that giant (maybe big, though). nevertheless, its not good looking and maybe there is a better solution. i'm on it.

a possible way for single sparks:

LoadSaveBlock(15610);

doesn't look that fancy but it will get the job done. maybe you could build the water channels smaller with insulator and zig-zag patterns.
replace water with NTCT and it would work for multiple sparks but the delay time is smaller due to the higher speed of the current.

i've update the creation. now, you can double (or even octuple [8 times]) the delay time by using flip-flops. so you dont net to lengthen your water channels anymore for larger delay.