To be exact, the current Mendix Application Container doesn't currently actually serve servlets, but sends requests straight through to the Runtime. It was also never meant to serve servlets (or third party apps), just be mean and lean and good at hosting Mendix apps.
If I were you, I'd just run a separate Tomcat container to run your own application, as it has a lot more configuration options to run your own app (again: the Mendix appcontainer is meant to run Mendix apps, not configure/run other apps). If resources are an issue, you should look into Jetty, as it's a really small container that performs amazingly well with a small footprint. (this, and the customization-via-code, are the main reasons we use it at Mendix).