Quick googling shows that /sbin/telinit -q will send the init process the correct signals to have it reread inittab. Note: playing around with init is not for the faint of heart. You should really know what you are doing or its possible to disable part or all of your system. There is also in the man page of init description of how to test init by running it in user space using an alternate directory as the root as far as the test is concerned.
Here is an excerpt from its man page:
/sbin/telinit is linked to /sbin/init. It takes a one-
character argument and signals init to perform the appro-
priate action. The following arguments serve as direc-
tives to telinit:
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0,1,2,3,4,5 or 6
tell init to switch to the specified run level.
a,b,c tell init to process only those /etc/inittab file
entries having runlevel a,b or c.
Q or q tell init to re-examine the /etc/inittab file.
S or s tell init to switch to single user mode.
U or u tell init to re-execute itself (preserving the
state). No re-examining of /etc/inittab file hap-
pens. Run level should be one of Ss12345, otherwise
request would be silently ignored.
telinit can also tell init how long it should wait between
sending processes the SIGTERM and SIGKILL signals. The
default is 5 seconds, but this can be changed with the -t
telinit can be invoked only by users with appropriate
The init binary checks if it is init or telinit by looking
at its process id; the real init's process id is always 1.
From this it follows that instead of calling telinit one
can also just use init instead as a shortcut.