At present, the only thing I can do using /tool fetch command is to make a GET call over HTTP/HTTPS or transfer files using FTP/TFTP. This tool is lacking a proper support for POST, PUT and PATCH methods over HTTP/HTTPS, which is problematic mainly because of it's impossible to do things the right way, like i.e. sending logfiles over HTTP PUT to a remote server.
There is also a more reasonable reason to implement this feature. As you can read, CloudFlare has deprecated their "normal" API calls used to set parameters like IP address on their hosted (D)DNS service. After November 9, all the RBs will be unable to update their records, which will affect dynamic IP users running webservers over their links. The only way to update the records will be to issue a HTTPS PUT or PATCH call with the new address, not a GET one.
Screw all the REST thing, but in my opinion, this will be a very desirable feature and it would simplify some scripting on RBs.