The solution to your problem is to use a divide by 2 counter. The ADC0808 can be clocked by rates upto 1280kHz. The ALE pin will give you a clock signal at 2MHz. feed this to the input of a counter and the output of this counter to the ADC clock input and job is done (the clock part that is!!). Anyways this scheme works. I have used it, successfuly (that is hard to imagine isn't it!!), myself around a year and a half back . I do not remember the IC number so I will not say more about that, but I am sure you get the general drift of this post:-)

Ofcourse I no longer use parallel ADC's. Too many pins are wasted. And if you are looking at using the ADC0808 to input data serially (to reduce the number of pins dedicatd to the ADC) then you could use a PISO IC, if i remember correctly that would be a 74HCT165. And then you bitbang...
This may be less expensive but you land up using more IC packages this way.

cheers,

Sushil