Why don't you just do what everyone else does and bolt a Garmin 696 on the dash? Dart Helicopter Services has approved mounts for a lot of helicopters.
Like Shawn says, it is all tied up in tech standards and approvals, so don't expect anything panel-mounted to change soon, or inexpensively. Even the IFR argument for idiot instrumentation is weak, with far better means of presenting information and improving safety with new displays (look at the SV stuff on the new S76D panel). Patience, it will come.
Don't most EC135's come with the big Garmin 530 GPS units anyway? That should give you everything you need - maybe not quite as good as a 696, but pretty close.
Have you checked out the iPad panel in the
iCub?