Maybe I misunderstand your question, but Caffe2 actually has the same C++ api to invoke inference (caffe2::Predictor) and is built from the same C++ source code on iOS, Android, Linux, Mac, Windows, etc. So it seems to be similar availability level to opencv dnn (please correct me if I’m wrong). You can check out the example android app to get the idea: https://github.com/bwasti/AICamera/blob/master/app/src/main/cpp/native-lib.cpp 44 We’re definitely working on better tutorials show-casing how e2e flow using onnx.
It’s a good point that wrappers for other languages like Java are not there by default, but we’re open contributions! 
In reality, if you’re talking about mobile deployment, there still would be differences in how you load your models (e.g. assets for mobile app) and potentially different optimizations you might want to do on your model for the optimal perf.
Another thing which might be interesting is to implement ONNX -> OpenCV DNN importer. It’s a bit lower priority on our list, but if you want to give it a try - please do, onnx is designed to be an open spec independent from the framework