I have Pronto codes for input switching buttons - if thats your problem - you do have to go about it in a roundabout way because as you probably know, the panny remote only has a single button for PC input and a toggle button to change inputs for the others, but you can do it.
Well actually you CAN switch between inputs on a Panasonic plasma using the Sky remote. I just set mine up after reading this thread. The HELP button switches between inputs on my Panasonic TH-PHW50BX. Can't change the aspect though.
The 'help' button seens to be the common button used to change input (its does this on my Sony, Panasonic and Goodmans portable).
What did surprise me is that the 'aspect' button for my 36" Phlips is actually the 'play' button on the Sky+ remote, so its possible that this will be the case for the Panasonic (although, I doubt it).
I have also found that one of the 'fasttext' buttons (red, green, blue and yellow) is often used for 'aspect' switching.