There is a workaround of sorts for this with two variations depending on whether you are setting dependencies for the relationships or not. Actually it relies on searching for the relationship, or to be more precise, something attached to it. This by default will show the topics at either end.
In the first option if you are setting dependencies then you can use the dependency icon, which can actually be formatted like a topic. All you need to do is click on this and apply any colour, for example a light yellow which won't change the appearance that much. You can then filter on the icon colour to show the dependencies and the topics attached to them (be warned that MM can sometimes do odd things to the position of relationship lines and the attached topics in filtered maps).
The second option if you are not using dependencies is slightly more complicated. Attach a callout to the relationship line (which is the only thing you can attach apart from a dependency icon). Insert a flag or other icon in the callout, then delete the callout text and remove the callout line and fill colour. This should leave only the flag or icon visible and you can then search for these using the power filter.
Unfortunately while you can save the formatted callout with its flag as a map part you can't paste it as a callout to a relationship line, but you can copy and paste it from one relationship line to another (I haven't tried saving it as a default callout yet). I've only tried this a few times and it works but sometimes there are a few odd effects - for example I was using flags and sometimes the flag colour would be changed arbitrarily when you pasted one onto a new relationship line.