New updated version is working now with the XE 10g edition.

The customizations are listed below:

1. Region Grouping Class Name - choice of regions going into RDS.
   Regions going into the RDS selector can be restricted by class name - put in the Region Attributes, e.g. class="rds-example"
2. Display "Show All" - option if the "Show All"-tab should be displayed or not
3. Label of "Show All" - label of the "Show All" tab can be changed
4. Escape "Show All" - needed in case your label contains html tags (e.g. <img>)
5. Css Attribute of "Show All" - you can set css attributes like title="your title" or class="class-name"
6. Position of the "Show All" - first or last, default is last

Example:

In the regions going into the RDS, set Region Display Selector to "Yes" - as you would with the standard RDS.
Additionally, in the Region Attributes, set a class of your choice (e.g. class="rds-example").
Repeat this for all regions you want to group in this particular RDS - important: the class name must be the same.
In the RDS plugin attributes, set the first parameter Region Grouping Class Name to the name of the class you previously put in the regions' attribute (e.g. rds-example).

You will find more detailed instructions on the demo page.

Credit goes to Martin without whom this plugin would be a half-solution.
I also owe huge credit to Jose Murillo for his thorough testing of version 1.1 of the plugin.
Thank you both again.