I found it was pretty easy to unzip the launcher jar file, use jd-gui to decompile org.spoutcraft.launcher.Main and save the source to Main.java, then add a line such as
params.add("-XX:MaxPermSize=128m");
in the reboot method. Then recompile using jdk 1.6 from the top level of the folder the jar file was unzipped into.
e.g. javac org/spoutcraft/launcher/Main.java
I'd post a jar file but that's against forum rules without admin permission. Hopefully this tip will hold people over until they release the new launcher.