May this advise change the way you think about the assets importing problem. Or not ... but I tell you, there is another way around.
Import and export is a headache, especially when you working with a lot of tools and imcompatible formats. The ugly truth is you can not have an universal way to beat this puzzle. I've work as the engineer for Gameloft specialize in tools, mostly in import/export and compress. We did convert a lot of things from Maya, max, blender, c4d... various kind of textures to varioys in-house format, compress, encoded configurations... you know that kind of job.
So what I learn is, (wait for it)... The workflow should be very, very, very, very simple but ...expandable. You may ask WTF is this kind of confusing riddle I raised here. For example Blender to jME as concerned here, what I did is to create Hooks (function that ready for extend)and Pipes (sequence or parallel convertions methods). That's the key.
In my Blender importer (my version is different from one of @Keathas), I create a lot of hooks, hooks when the blender file reader touch a Mesh, a Texture, a Node, a Shader, a Contraint... So the importer take a configuration file (like the one you have here) and work like an irterator over interested nodes. The hook will be process through a filter (a condition) to see if its need futher process, for example: we only want to import the floor object. Then the data pushed into the MeshReader for example to procedure a jME Geometry and saved it into ConversionResult (in memory). The pipe can be continue as I let the jME spatial be saved into a file in a folder, ziped it, push to git or Drive... The whole system need an Assets server which may be run from the jME SDK, but also can be run from gradle or maven because it's actually a jar. More amazing our team managed to integrate also Nodejs stuff to convert and push packages online, procedure an delta for Mobile device to download lastest version. And do it automaticly with Jenkin
So what you doing here is called a Scriptable Asset Server. You can use Gradle as the Scriptable part and should write another Blender importer (like i do, its pretty simple) with hooks.
You can also wait for me to release the AtomAssets workflow (again), which actually opensourced in the past but then removed because we trying to do commercial thing with it but un successfully