I figured it out! Luckily.... As I posted above, using the SuSE installation disk. I chose Installation--->Other Options--->Repair Installed System--->Customized Repairs--->Start Partitioning Tool. The partitioning tool in this case is Expert Partitioner, just like during initial setup of the system. I was able to edit the /opt volume size without a hitch. But here's the tricky part:
After the edit, it is necessary to Accept the partition scheme. Same way as during initial setup, except that all the mount points are gone! And they can't be reassigned! Then a dialog appears asks to assign a swap partition! All this stuff was already nicely arranged in my system. But, I took a chance and ignored everything. Luckily, the /opt volume was resized, and I was able to exit Expert Partitioner unscathed.
I found the process very unintuitive because it left me with the impression that the system would get messed-up with every mouse click. Maybe a sticky on this topic would be a good idea. At least until EP's dialogs can be updated to instruct users better.