I think Jim Foltz made a rotate camera plugin ...
Rotate view (camera) by 90 degrees - #3 by john_drivenupthewall
The link FTR, is: Quick Camera Rotate 90 (Roll)
Not sure if it works in Top View Parallel Projection.
Ah okay. Got it now. Yes other users have brought this up in the past and asked for a way to lock eye height.
A plugin could do it, but the Pan tool should be modified to do this, say with a SHIFT modifier.