Scriptable extensions are definitely planned. It is tracked by the following issue:
The current plugin system is largely unsuitable for custom additions, even for adding import/export formats. This is because they require binary compatibility (usually means they need to be compiled for the same platform, with the same compiler and the same Qt version as Tiled), which essentially means you're already compiling Tiled yourself at which point you can just add your tool to its C++ code. The only benefit of having the file formats in plugins is that they can be entirely disabled.
Of course, a C++ plugin would be the only way to expose the full functionality of the QGraphicsView and QPainter to the tool developer, though in the interest of future compatibility and comfort it may anyway be a better idea to provide an API in between.
Regarding your particular tool request, it's actually a common request and tracked by the following issue:
I had hoped to work on this sooner, but at the moment I have very little time for new projects since I'm mostly busy mentoring the three GSoC students working on Tiled (who have chosen various other projects).
The latter thing is not going to happen, since it would essentially require an XML document to be represented and this could then only be supported by the TMX map format and not by the JSON or Lua formats, which currently do support all features of Tiled. However, enhancing custom properties with structures and arrays is certainly planned (#489 and #1493).