<maven:maven/> tag should be able to inherit at least a subset of variables.

The maven:maven jelly tag should be able to inherit at least a subset of the invoker's environment in order to do controlled override of properties set in build/project properties.

Either by using something like <ant inherit="true"/> or by context or hierarchial naming convention of properties, ie:
my.prop1=a
my.prop2=b
your.prop1=c
...
<maven:maven inherit="my.*"/>