The idea of defining a "magic word" that marks a custom makefile seems not straightforward to me.
I have two ideas for that :-)
First:
I would rather add an additional (optional) parameter like the following:
<Option makefile_is_custom="1" custominvoke="nmake" />
Second: (which i'd prefer)
Add the possibility to choose whether to compile via Makefile or a compiler configuration by adding a "makefile" option to the target configuration:
<Target title="MakeRelease">
<Option invokemake="make" />
</Target>
This would give us the option to have several different compiler configurations AND also a custom makefile.
What do you think?