Capping CPU usage?

Folks:

I had a new question from a client: is it possible to "cap" CPU usage
 for PostgreSQL running on Linux? They don't care if the procedure
 degrades Postgres performance, but they can't afford to have Postgres
 take up more than 30% of processor for more than 400 milliseconds
(they
 are running some real-time operations).

I can't imagine that postmaster could do this, but I thought it there
 might be some kind of Linux Kernel CPU quota option I haven't heard
of.
  Can anybody point me in the right direction?

-Josh Berkus