In Windows explorer you can access the folder properties and change the icon folder. It appears to not follow if you reboot the pc or change it. Is there a way to make it by the Directory Opus folder options? For example, that would integrate something like a source folder (for the icons) and the path of the folder for wich the icon is changed. It could be backed up and restore on a new pc or after system restoration.
As an aside, the same Properties dialog can be accessed from Opus. (Literally the same dialog; we just tell Windows to display it.)
Customisations made to folder icons in that dialog should save between reboots. If they aren't, there's probably a permissions issue on the folder preventing the desktop.ini being written inside it, or something is either removing that file or removing the R (read-only) attribute on the folder. (The R attribute has no real meaning on folders, so Windows uses/abuses it to indicate that it's worth looking inside for a desktop.ini file that might customize its icon and other details.)
(If the folders are network drives then there are other complications as well, but I assume we're talking about local folders.)
Opus also allows you to change icons via its labels system, which you could use as an alternative. But those changes won't be visible outside of Opus, while the ones made in the Properties dialog are, so I would try to get the Properties dialog method working first, unless you only need to see the icons in Opus.