By setting the JSON lognode to trace you will be able to see all client/server traffic. Please note you will incur a performance penalty when doing this because server responses can no longer be streamed to the client. Also note this will record passwords as well.
With this information in your log file you could rather easily write a client in some programming language to simulate multiple users. I have written some basic implementations of this as a Java application for a handful of projects but there is no standard solution for this. It is not as easy as simply pressing 'play' on your recorded traffic because then you'd always be going through the application with the same data, so things such as randomly selecting an item from a returned list of objects (such as when you click an item in a datagrid) should be programmatically implemented as well.
You could also have a look at frameworks such as Selenium.
Don't know about some testing tools, but if you just want to do a load test, you could create a microflow that does "things". You could trigger (webservice call) this microflow multiple times in order to test the load. But I think you want to simulate users clicking at buttons and opening forms I guess.
We are looking into creating a load test with Scapa software. This tool records communication by setting itself as a proxy between de browser and Mendix.
A first trial showed we need to work on the recorded communication to be able to variate the test script and to introduce variable where a created object can be used in further calls.
One good script can be made in say 5 days (by a Scapa specialist, not me) and then the scapa software can generate any load you want.