In FlexMojos 3 there was a goal for taking an installed swc artifact and installing a swf that could be used for the RSL.  The goal was called install-rsl.

Here is a sample execution:
mvn org.sonatype.flexmojos:flexmojos-maven-plugin:3.9:install-rsl -DgroupId=org.as3commons -DartifactId=as3commons-lang -Dversion=0.3.1 -DoptimizeRsls=false 

This requires that you have the swc installed in your local Maven repository.  Once complete you will have the swf installed on your local repository.  You can then deploy this to Nexus.

UPDATE:  I revisited this recently and ran into some difficulty which I assume was related to now using a 64-bit JAVA SDK on my machine.  I ended up doing the swf creation manually using a 32-bit JAVA SDK and the actual binaries that shipped with the specific version of the Flex SDK I was using.  Great instructions for this at FlexMojos Wiki