one possibility, although very limited, is that you could find the slope of the line, and use that to your advantage... to find the slope, start at an X value, and look where you line crosses the Y axis. Then move to the right, one space (X + 1) and check where the line intercepts the Y axis again. Try starting where the line crosses the X and Y at the same time, and then go up, or down, the line and see where it crosses the X and Y axis again... Once you found 2 spots where they both cross, you can find the rise/run = slope...? uhh... that might be abit hard to understand, so I'll try and take a pic of it.
alright, here it is. the 2 lines have differn't slopes, and you can clearly see it. on the first line, there is a rise of 3 and a run of 2. At the green points is where you could put your new vertices for the intercepting line, since it is directly at a cross point(x=a,y=b)...
This is a very limited since it won't always be easy to find the slope of lines... hope this helps though
I seriously have no clue as to what in the world you are trying to achieve. If I understand you correctly, you just want to have some outside vertecis line up to a face. What you could do is just select the top and bottom vertecis of the face that has the incline you want and copy them by Shift + Click + Dragging them.
This is something i never found a real solution for in max, infact i dont know if there is a proper one, simply enabling snap to edge is no good because then it will effect the y value aswell as the x, and enabling use axis translation so it only effects the x seems to dissable the snap from working? . So to fix this i often had to use roundabout solutions like what you suggested before another method to try is to align the cut plane to the edge and use that to cut the other object so it will be cut in the correct spot then you can just use target weld. (if that makes sense to you).