I am not sure what is your requirement for standardized RPC communication method over MQTT but I still made a little search around and found several points to deliver to you.
First of all, about the standard method. On one Github rpc project the description kind of nails it:
This module provides an rpc interface for an mqtt connection, in essence this is a request and response strategy which uses an MQTT topic structure as transport.
The point is to use stack model (OSI model is a standard) and just use MQTT as one of the stack's layers.
This project on json-rpc over mqtt uses standardized or at least well known protocol over MQTT.
About awkwardness, you do not have to build the MQTT and the RPC on top of it from scratch but instead use one of the linked libraries or some other found on Github.
OK, for every language there is no ready basis on Github, but in that case the compliance becomes a planning criteria and may affect the whole other project, unfortunately.