I found it difficult to get the exact result you want but if you tinker a little it is possible to come close.
Option (b) - the relationship of a boundary to itself - is the easier to mock up by creating a relationship between a the boundary and a subtopic within the boundary. You can then format the relationship line to be something like what you want (see Figure A below). It is impossible however to completely the stub of the relationship line inside the boundary, though you could disguise this by giving the boundary a solid fill colour.
Option (a) - relating a topic to itself - is slightly more difficult but amounts to the same process. You can create a boundary around a single topic and then establish a relationship between the topic and its boundary, but then you need to make either the topic line or the boundary line disappear by making them either white or the same colour as the map background. Again however you end up with either a stub inside the boundary line - or alternatively a gap - depending on which approach you adopt (see Figures B and C below).
You are right regarding the requirement to connect two different objects but I just thought of another workaround - a bit more fiddly but it ends up looking more like what you want.
Paste a floating topic at the furthest corner you want the line to extend from the topic or boundary concerned and put a single space in it so that it is blank but visible.
Then run a relationship line from the topic to this floating topic and format it as a right angle. This will be half your final line; to form the other half, insert a relationship line going the other way, from the floating topic to the topic and add an arrow end style as required.
Format this as a right angle as well then use the anchor lines on this and the first relationship line to ensure they form the square shape you want. To do this you want the same connecting point on the floating topic for both lines but different ones on the actual topic - usually one on the side and the other on the top or bottom.
Once you have got these lines the way you want format the floating topic to make it invisible by selecting no line or making it white and removing the fill colour. The result should look something like this (I've added a callout to indicate the location of the invisible floating topic):
MindManager does not do this with a standard function, so every solution will be a bit cumbersome.
The first questions that pop up are.... Why do you want it? How will you use this?
So, when you share your reasoning with us, we may find a completely different response that fits your needs