I wish it had that capability too.
I created a script that checks for (among other things, such as the current server load) the number of currently executing processes of a particular name. (ps | grep "name" | wc -l or something like that) If the count is above my limit, the script goes to sleep for x seconds before waking up and checking again. I make that script the first step in the jobs that I want to limit, so that while multiple might be submitted, only approximately my limit will really be running. Approximately because there's timing issues--if the active count goes below the limit and there are multiple jobs waiting, two jobs might notice it at the same time and release, so that there are now limit+1 jobs running. Also, the jobs don't necessarily run in order that they were submitted. And you can't tell from rundeck (without going into the job) that it's really waiting in the script because from rundeck's perspective it's "running".
Also, if you submit lots beware of the default limit of 20 concurrent jobs in rundeck. I bumped that to 99--there are times where I have dozens of jobs waiting to execute.
If you're submitting onto a different server this would also be problematic.
So... definitely not a perfect solution, but it's something to consider.