Power supply fix for Canoscan 8800f
ThreePhaseEel's design and Whit3rd's version appear to only be theoretical. Considering the lack of information on why this 'short the resistor after a few seconds' trick is needed, I would stick with the relay based time delay circuit since it is known to work. 

But first I would try just wiring up a 1k5 resistor with a mechanical switch across it. From this I would find out whether the technique works at all, and what time delay is required. And if the timing can be reliably done by hand then why bother with a more complex circuit?