Most haxe “native extensions” are usually about providing iOS or Android API to Haxe and you want to do the other way around so you are mostly on your own building a solution. That said:
You found my experiment (hx2objc). It is probably very incomplete, but the proof of concept worked. I think it is possible to create a macro that generates bindings to plug Haxe/C++ to iOS through Objective-C++ files (and Objective C header which is Swift compatible as well).
On Android, the same technique can be used to generate JNI bindings (but you also have the option to simply create a java library from Haxe depending on what you are doing).
So, to answer your question “are there any showstoppers on the way”, I would say no, it’s definitely doable and can work very well, but it’s not easy to setup because the most viable option in my opinion is to plug Haxe/C++ generated code to iOS/Android so you need to have good knowledge of C/C++, how Objective-C, Objective-C++ and Swift can interact together (as well as Java and C++ on Android) and you will also need to take a closer look at how to play well with HXCPP’s generated C++ to make Haxe <-> Objc conversion or Haxe <-> JNI types conversion. That’s basically what I started to do on iOS with my experiment “hx2objc”.
I believe haxe is a very good option (and a good alternative to “raw C++”) if you need to create a big (non UI) library that will be shared between iOS and Android. I used to work on a big codebase that would have benefited a lot from this solution, but at that time I didn’t know Haxe and HXCPP well enough to do so. If today I were in the same situation I would probably consider very seriously this option and build something up. (Un)fortunately I don’t need any of this anymore on my current projects, so I stopped exploring generating bindings from Haxe. I however did the opposite: parsing objc and java files to generate haxe-compatible interfaces, which requires similar knowledge, and may be helpful to look at if you want to see how types can be converted between Objc/Swift and Haxe/C++ (especially hairy cases like converting Objective-C blocks to/from Haxe callbacks ). There are also example projects that demonstrate how to run HXCPP code in Xcode project and Android Studio project, which is basically what you would do when plugging a Haxe library to iOS or Android.
I would also add that building such solution should not depend on anything like lime, openfl, nme… That’s mostly a “binding generated C++ code to iOS or Android” problem (which doesn’t stop you from using these after, but that’s another story with its whole load of problems to solve )