Is there a method to list all the categories of the system without it being associated with a node?
?func=attributes.dump this information comes from the table CatRegionmap and a Join to Dtree for subtype131
Any Table in CS can be shown through a REST API by a WR and a LR combination
Is there also a way to get this dump in xml or json format?
I would hate to repeat myself that Request Handler runs a simple DTREE /CATREGIONMAP query, if you are an Oscripter you can create a rest api endpoint or ask OT for a feature request.
If you are in a hurry then a WR/LR can be made and used adhoc.
I can think of couple of ways to get all categories in the system via REST API
Search might be better as OT has no problem creating categories other than the category Volume