Any news on this?
I tried to compile the code on github, but it failed.
The error makes me think the code is for KDE4. I don't have KDE4 anymore, hence the error.
I think today it would make more sense to build something for KDE Plasma5, though I understand that in 2015 (the date of the code on github) that choice could make sense.
The dolphin-plugins package has recently started providing support for emblems in the Dropbox folder. You may want to look at the code to add support for Insync too.