#1
A band i've been asked to produce demos for wants to use this effect at the start of a song. I think i know what it is but i wanna see what other people can recommend.

http://www.youtube.com/watch?v=danEuXSTjMw

I think it's EQ automation. Cutting off the high frequencies and gradually shifting the cut from highs to lows, then bringing all the frequencies back into it. Does that sound about right? It's only a rough guess.
#2
that would be eq automation yes, sounds like basically that cut it down to a thin low end spectrum then slowly raised it up to the lower/low mid areas and then automated it up to whatever the master eq setting at that point was.
#5
It would be done after the process of recording everything . Just apply the effect to the master instrumental track so it effects all the instruments.

You could loops the audio from computer into wah back into computer and record, but it would be a lot easier to just a filter plugin.
#6
Using a filter would be the way to go so that you can get a smooth automation and make any necessary adjustments. If you were to use the wah method, you will need to make the wah adjustment perfectly during playback or else you would need to start over again.
#7
Add a low pass filter, then automate the filter to open up progressively (if that makes sense). For example, in Logic it could be done using a plug-in called 'Fuzz Wah' then if you go to the automation and select to automate the pedal position, you can have it from one end then automate it to open up as the song progresses until it reaches the normal state.