How do I circumvent the versioning system to run an outdated MacOSX application?
If the actual executable indeed runs without problems 
then it looks like it is just an enforced blacklisting for whatever reason.

In the .app-bundle there is a file 

/Applications/iMovie.app/Contents/Info.plist


In there you find the variable (below corresponds to v10.1.6 of iMovie):

&lt;key&gt;LSMinimumSystemVersion&lt;/key&gt;
&lt;string&gt;10.11.2&lt;/string&gt;


Edit this to your liking.

For the other way around, this is reported to be working:

Look in the same file for
    CFBundleVersion

&lt;key&gt;CFBundleVersion&lt;/key&gt;
&lt;string&gt;6.0.4&lt;/string&gt;


Change 6.0.3 or 6.0.4 to any other (higher) number.  

Changing it to 6.0.5 seems to work fine.
You might change it to something higher than that in case Apple decides to disallow other 6.x.x numbers in the future. 10 (the current iMovie major version number, no .x.x needed) also worked just fine.