I'm at home now and don't have SW here so I can't post an example, but I'll try to explain it.
Assuming that the spring is modeled from a U or V-shaped extrude, I would place a construction line in the sketch connecting the ends of the straight sections (where it will contact the glass). Constrain one end of this construction line but leave the other end un-constrained, and constrain the rest of the sketch so that when you drag the un-constrained end the sketch will move as desired.
When you insert the spring into your assembly, mate the fixed end of the construction line to the glass and place other mates as needed to keep the spring from rotating. Next, edit the spring within the assembly and make a coincident relation between the end of the construction line and the other piece of glass. The spring may not move dynamically with the glass but it should update when you rebuild the assembly.
While I'm writing this it occurs to me that it might work better to have the construction line as a seperate sketch in the part, with the sketch for the extrude below it in the tree and restrained to it, but I realy don't know which would work better.
Thanks Glenn & Deepak for the help on this. Very much appreciated!