Assume the service you want to start as another user is "myservice", here is what I would do:
mkdir /usr/local/init
cd into /etc/rc.d/init.d
Move the myservice init script you want to change the user for, into /usr/local/init
Make a simple "wrapper" shell script to execute it as a different user, and name it what the service is
Code:
#!/bin/bash
exec sudo -u USER /usr/local/init/myservice
On boot, when the service runs, it will run your wrapper script, which executes the real init script as the specified USER
Hope this helps