One way to do it: Make center of line for slot coincident with hole center.
If I'm understanding correctly, the easiest way would be to use the slot feature to model the slot in the part and then use the Axial Align relationship in the assembly. The Axial Align relationship will recognize the slot axis as shown in the image.
I guess I would have to remodel parts to get a slot feature. I don't have the luxury. But I like the idea.
The sketch midpoint idea looks like it will do.
There is no such thing as an axis feature in SE?
Greg's solution is for after you generate the slot feature (better for synchronous because the sketch becomes useless after), mine is for before inserting the slot feature and would sustain in an ordered mode.
I always thought it was a little weird that SE does not have an axis insertion capability and only has implied axes.
I am glad to hear you say that bshand...
"I always thought it was a little weird that SE does not have an axis insertion capability and only has implied axes"
The best work around I can think of is to use the Center-plane relation (single) twice. Once fron the CL to the sides, and again from the CL to the ends.
Expanding upon your sketch solution, you could also use an axial align here instead of making the hole center coincident to the midpoint of the slot sketch. Axial align will also recognize keypoints of sketch elements to create the inferred axis, so you could simply align the axis of the hole cylinder with the inferred axis generated through the sketch midpoint. The one advantage to using this method instead of the coincident method is that you can then move the part along the axis if you need to...