While many desktop readers have this feature, I own a Kobo Glo and this option doesn't seem to be available.
As you probably already know, CBZ and CBR files are just plain standard zip and rar archives with renamed extension. On their inside, the comic pages are numbered image files, which the reader simply shows to you in alphabetical order.
So, in example a typical content could be
and so on. If you have comics that should be read in R>L order, I think that your best option is to rename the image files to their correct display order, so, in the previous example, page01.jpg should be renamed to page02.jpg and viceversa, page03.jpg to page04.jpg and viceversa, and so on.
Doing this manually can be an annoying task, but surely if you put in a search engine "file renamer" plus your OS, you will find a lot of tools that can assist you in this task.