Hey all!
I’ve written an extension which will make Yii2 a JSON-RPC server. I didn’t like existing ones because they don’t really use Yii2 power.
Mine extension would translate “method” to Yii’s action using routing system and then call that action and wrap results into JSON-RPC compatible response. Therefor you can use all Yii features such as filters, behaviors, etc.
Besides that I wrote an extension to be fully compliant with JSON-RPC spec. And yes, it supports multiple requests in one call.
Please find it on GitHub, name is georgique/yii2-jsonrpc. Rules here on the forum are a bit repressive, so I can’t post a link to it. 
Note that it’s still in development, and I am actually posting it here hoping somebody gets interested and might do some testing and provide feedback.