The EC2 plugin works great for dynamically provisioning EC2 slaves (as needed) when their label is specified under "Restrict where this project can be run," but it doesn't do anything if that label is passed in using a NodeLabel parameter. Note that this only seems to affect the EC2 provisioning step; jobs set to run on a slave via NodeLabel parameter will happily run on provisioned-by-EC2-plugin slaves that are already around.