After some thinking, I found a way to add XnView 1.98.0.0 to the context menu that appears after clicking in the empty space of a folder\desktop (the View, Sort by, Refresh... etc menu).
It's done by adding a XnView key here: HKCR\Directory\Background\shell, with the command "PathToXnView\xnview.exe" ".\"
That would result in XnView opening in the current folder, without the need of right-clinking over a folder. I find it way easier, probably because when I right-click a folder, the context menu has just too many items: my AV, 7zip, Take ownership, Open Command prompt here, etc

Anyway, back to XnViewMP, while the normal context menu integration works fine, that other method results in XnViewMP not loading completely, it shows the window dimensions and title bar normally, but it doesn't load any menus or icons, it's all blank and flat.
Now I understand it's in Beta stage and is unstable, so no need to jump on me because of that. I am only posting this thread to ask for the person or team responsible for the development to at least look into it and if possible, make it work like the 1.98.0.0 version (as I don't know if anyone could say that's caused by a "bug" or simply because XnViewMP is just that different from the older versions).
Thanks in advance.
EDIT:
Wow, so long ago!
Update - Using the same method, but with this command instead:
PATHTO\XnViewMP\xnview.exe "%V"
As of 0.61, it works just fine! You can launch XnViewMP through the context menu when clicking on a empty spot in the folder.