Taking the hint from a comment by lufthelfer, I did some adjustments to the extensions and submitted them to the Opera Add-ons store. They have some strict policy for translated descriptions. But the extensions got through and are just published!
The 3 extensions from the 'Simple' series are now available for Opera!
On the side note, there have been minor bug fix releases for SimpleUndoClose and SimpleExtManager. (Thanks to Emm for discovering the SimpleUndoClose bug!)
Nothing major for the time being.