@Szabolcs I might have a simple solution for the IDEA problem of connecting Mathematica to it. I'm not sure how easy it is for me to implement this, as most of the MathLink programming is trial and error, but here is the idea:
Inside IDEA, i.e. the plugin, I open a KernelLink. This can be used for evaluating stuff as we want. In addition to that, I simply open an additional MathLink with a specific name. When you add this link as kernel config in Mathematica, the FE can connect to it and I can pass all traffic through to the kernel. I'm not sure this works with an already open kernel nowadays, but at least in Wagners MathLinkMode this is done as well.
