Automatically set keyframes to start/end at clip start/end?

I would create the animation using an expression that looks at time-inPoint and outPoint - inPoint, along with a couple of linear or ease interpolation methods. You can set the duration of the animation, the starting and ending values of the first two keyframes, and the starting and ending values of the last two keyframes. 

If you wanted the scale to start at 90, 10 and animate to 100, 100 over 20 frames starting at the layer in-point, then go from  100, 100 to 75 , 0 over 20 frames at the end of the clip, the expression would look like this:

t = time-inPoint;
animIn = framesToTime(20); // 20 frame starting animation
animOut = framesToTime(20); // 20 frame ending animation
stVal = [90, 5]; // Starting Scale
endVal = [-40, -80]; // Calculate ending scale as 60, 20
animEndStrt = outPoint - inPoint - animOut;
// Animations
sclUp = easeIn(t, 0, animIn, stVal, [100, 100]);
sclDn = easeOut(t, animEndStrt, animEndStrt + animOut, [0, 0], endVal);
// Combine the animations
sclUp + sclDn

When you get the animation looking the way your want it to, save the animation as an Animition Preset so you can easily apply it to dozens of layers at the same time.

I've probably created about 100 of these kinds of expressions to automate animations. Most of the time, I also incorporate some of the basic principles of animation, like squash and stretch, anticipation, staging, and slow in and slow out, and others as taught in "The Illusion of Life" by Frank Thomas and Ollie Johnson—a book that anyone creating animation should read about every couple of years.  Everything you see here can be automated using expressions: