First, just to be clear, the OSD in MC is displayed at the bottom of the screen when a video is playing and you press the up or down arrows on the remote. When you move your mouse to the top of the screen with a video playing, the MC "Media Controls" drop down. The Media Controls include an active progress bar, which updates as the media plays, and can be dragged or clicked on to move to another part of the file.
1) I am using MCC command MCC_PLAYBACK_ENGINE_SET_ASPECT_RATIO, -1 to cycle through the aspect ratios as a video is being played. Is there a way to have MC display the aspect ratio in the OSD? (e.g. 4:3, 16:9, etc). Right now, when I cycle through the aspect ratios, I have no clue what aspect ratio is being applied (obviously, other than visually observing the display).
Why don't just use the OSD to change Aspect Ratio menu item to change Aspect Ratio? That way you will be able to see which Aspect Ratio is selected, and which you are changing it to. You will also be able to see the effect of the changes real time, as with the MMCommand. Once you have made changes this way, the setting is written back to the "Playback Info" tag and used in future whenever the file is played.
1b) A more general question about aspect ratio - does MC automatically play each video using the source aspect ratio?
If you have the setting under "Tool/Options/Video/Advanced/Aspect ratio correction:" set to "No correction", and you have made no specific Aspect Ratio changes for that file, then the file will be played back at the original source Aspect Ratio. Of course, your TV could also make changes to the Aspect Ratio, if set to do so.
2) for movie descriptions, is it possible to have that field maximized by default so that I don't have to click on the field to expand the text box?
Not with the default functions, but if you really wanted to do it, I'm sure it would be possible to modify the MC Skin to do so. That isn't particularly easy though. Search and research how.
3) is there a way to call up the OSD (progress bar) while a video is playing? I see that the OSD appears when I pause or skip through a video but it would be nice to call it up on demand during playback
As above and previous answers, press the up or down buttons on the remote. Keep pressing the button to scroll through menus and displays. The menus change based on the file attributes, whether Subtitles are available, etc. The remote Info button shows a slightly different display, but you can still scroll to the others from there. Not that this progress bar is a snapshot of the current position when you press the button, or scroll to it.
4) is there a way to modify the OSD to display more information? Right now I only see a progress bar for the current video.
By now you should have worked it out: Keep pressing the up or down buttons.
But also I have changed what the Info button does on my remote, so that it sends the MCCommand /22001,2, which jumps to the Theatre View Playing Now screen. From there I can use the menu to view the file's Info page, or navigate anywhere else to check information, and when finished if still on that menu, just select "Display View" to go back to the movie, or press the "Media Center" (MCE big green button, or a button with the Media Center command assigned to it) button to toggle through MC display back to the live movie. If you didn't pause the movie, it would continue playing and be visible on the Info display.
As you are using Girder you should be able to add the Playing Now command to a new button, if your remote has a custom button capability.