Thanks for the comment. Reference to the noted page shows that aspect adjustment on this set is available for 4:3 sources only. It does not function for 16:9. This fact was noted in the CNET review of this model Panasonic TH-PX50U series (2005 plasma) review - CNET This fact seems to be often lost on owners of this set, who report on some of the forums that the feature is "greyed out". It is only greyed out when they try to adjust a 16:9 source. It would work on a 4:3 source.
Unfortunately, it appears that there is no solution that I have been able to find for the noted problem, other than to tolerate the sides of the picture being cut off. Since the Radeon HD software is able to scale the picture for its card on my other machine, any chance that feature could be added to the Intel Graphics software so that the higher interlaced resolution of the Panasonic plasma could also be adjusted?