Thanks for your fast reponses. My lengthy response on that got lost due to a forum timeout

So this one I'll keep short.
How is that done, does Windows Explorer respond to the USB 'device connected' event ?
If you are connected to multiple cameras, can it distinguish each one ?
Certainly, it does. Gives you a symbol for each cam, which you can open to view and copy contents. It also gives you the vendor guid in the property windows.
How do the Powershots respond to WPD_COMMAND_CAPABILITIES_GET_SUPPORTED_CONTENT_TYPES ?
Please have a look at the attached screenshot. It should answer most of your (even unposed) questions.
Since the WPD COM-objects functionality is mostly undocumented, especially when approached with managed code (C#), I currently have some problems on uploading content, while downloads with jpgs and movs (haven't testet other content types right now) work perfectly.
Have you verified that your camera will actually do this?
Gosh. You are right, my Windows 8 Explorer reports an error, when I try to do it with my A2400IS. But I remember having done it with a Windows 7 machine on some powershot type.
But does't your a540 provide enough evidence that file injection can be done in principle? I'am not so experienced with Lua right now. But if Lua can access all the storage on the device it should be able to read it bytewise and copy some "special" content into script files or interpret it some how.
What is my project?
Currently I'm running a large cam array using a uBasic script and periodically collect the data over USB via WIA. Since WIA doesn't read videos (since Vista) I'm currently reknitting my code and solution to use WPD. New possibilities give new ideas...
What is my goal?
I want to be able to update automatically either a script or at least inject parameter files, or some other interpretable stuff without the need of SD card removal or proprietary drivers.
Best PeterT