as far as the green wire goe's for testing that's ok,for full time run no not a grand idea,as it is the power supply portect line
yes, if you ground out the green line it will turn on the power supply, this will let you know at least the powersupply is still functioning okay at a rudimentary level. (you mentioned the fans spin up but nothing else.) while it doesn't eliminate the power supply as the fault completely, it's enough to move on to something else.
you are going to have to find the power switch header. I don't recall 100% which board was used in the uprights, but i believe it's probably the ECS board. If your board is actually one of the brown ECS boards (P6IWP-Fe)
the front panel connector is the top rightmost connector in this photo. it has 21 pins (one missing for indexing purposes)
if you use a pen or screwdriver to jump the 2 leftmost pins as oriente in this picture (one on the top row and 1 on the bottom row), your board should turn on. if it doesn't, it may be toasted.