How difficult is it to adjust the tilt fast enough?
I would put it like this: just guess, and you will probably be far superior to not tilting.
Personally, I'd shoot at f/16 or something with the 90mm/2.8 and the EF2x. and a 250D or 500D closeup "filter" lens. I'd choose a tilt, then manuver the camera until it seemed to maximize sharpness in the viewfinder at f/2.8. Granted I couldn't see fanstastically well how focused it was, but since the actual photo was stopped down 5 stops more, if it looked OK in the viewfinder it'd be good in the image.
I generally shot macro that way: adjust the camera, then move back and forth until I nailed the focus, so you could say I handled tilt the same. If it is really to near/too far/too tilted/not tilted enough, then I'd adjust a bit and try again.
There's also a "hinge principle" that worked on old view cameras and I kind of mentally picture in my head to estimate what will work: Picture the plane the film is located on extending infinitely in all directions. Then picture the plane of the tilted image, interesecting with the first plane along a line. You adjust your lens so a plane going through the lens's front element also has to intersect the other two planes along the same line. To picture this, open a hardcover book on your desk about 45 degrees and stand it vertically. The back cover nearest you has the film mounted on it. The front cover is the plane of focus. Some page in the middle of the book has the lens going through it. Now I don't know exactly whether the lens plane is meant to be the front element or middle element or what, but, if I just use this as a quick rule of thumb for my first guess, it's often something I can work with.