Wat. This isn't possible, the plugin will work for whichever version of Java it compiled on and later. No other plugin can change that.
Erm, first of all, why? Why not just update Java. Secondly, as Darq stated, it can't be changed post-compile time.
I have G5 mac 10.5 java 5, java 6-7 does not run on this mac therefore all plugins will crash or make errors in a minecraft server but I may have found out an alterative called Soylatte a Java alternative.