You are free to use "system.describe" as any other usual method name, in JSON-RPC 2.0.
If (and only if) it is for local standardization purposes, you can to use the prefix method name rpc.
(ex. "rpc.system.describe"), see Extensions section,
Each system extension is defined in a related specification. All system extensions are OPTIONAL
Help yourself, about "defined in a related specification", you not need an international discussion or consensus, it is also for local/personal use.
This kind of "local standardization" is interesting in a context of multiple endpoints (or many local web-services), and some common and controlled options to check endpoints.
Notes
Another question is about JSON-RPC 2.0 itself... 2.0 specification is alive? It is RESTful-compatible? How many systems are using it, there are some statistical data? ... I am using it, but with this kind of questions :-)
Uniform interface, with self-descriptive messages, is a good requirement, that can enhance JSON-RPC pack (eg. by use of JSON-LD), but there are no discussion since 2013 (perhaps 2010).