The system I propose for the multiplatform award is this:

Assuming we allocate 2 N9's for the multiplatform award
1. All other awards are given like we didn't have the multiplatform awards
2. Each runner up in the 7 categories (7 applications) compete for the multiplatform award. The two with the most supported platforms get the prize.
2b. In case of a tie (e.g. 4 platforms for the first and second application) both get an award
2c. In case of a tie in the second-third place (3 platforms for the second and third app) then the winner is the one with the greater absolute number of votes.

Now for the platforms that compete I think it should be any platform Qt is running fully on, and clearly stated from the start.

Harmattan
Fremantle
Diablo
Mer
Symbian
Linux Desktop
Windows
Macintosh
Blackberry
Android
anything I forgot

The reason for including all platforms is to create more possibility for differentiation (so that we don't have 90% of the apps target all the platforms) and because the real challenge is to target an entirely different platform (like symbian) and not just linux flavors.

I deliberately left iOS out as a little research showed that Qt is in a very early stage on the iPhone, whereas on android, we have got a complete SDK with the equivalent of smart installer for easy deployment.

As a possibility we can award double points for maemo platforms if that does not make the system too complicated.

This system gives incentive to all the contestants to target multiple platforms as they have more chances to win, and promotes Qt best coding practices.
This means using Qt wherever possible and ifdef'ing platform specific code which will make later porting to whatever platform easier.

It also adds a little strategy to the mix as a developer has to choose between investing time to port to other platforms and polishing the app with little useful features. The first might award him the multiplatform award but the second might push him up a place in the rankings.

Q's and A's

Applications have to fit to the other categories so this becomes void. (if an app meets the criteria to enter any category it automatically competes for MP award)

See points 2b/2c

All platforms that enable the developer to deploy the application with an installer. (Meaning no terminal trickery or manual copying of files required from the end user). Also a ready made SDK is required. We can't expect the developers to have to compile their own Qt.

I think this solution is not overly complicated (2c is not so probable) despite my lengthy description.