If boot parameters are allowed in lilo, you can drop directly into maintance-mode(=runlevel 1) by booting "linux 1" (that is if your bootloader (lilo/grub)-choice is named as linux).
Some distributions (I don't know about redhat) have yet another root password query at the point of dropping into the maintanance mode, so it might have to be done in the hard way.
"linux init=/bin/sh" should drop you into the prompt where you can remount the root-partition to readwrite-mode
mount -o remount -w /
and run the "passwd" command.