I too have had no luck with finding info about editing .qmg files, as far as I can tell the only option is to swap them.
After much swapping with poor results because the files I found were for phones not tables..
I found that I could copy the second part of the bootanimation bootsamsungloop.qmg
and rename it to bootsamsung.qmg to replaced the first part of the bootanimation
so that instead of an ugly orange Telstra swirly logo and then the Samsung logo, it was just the Samsung logo.
Then I did the same for shutdown.qmg to get rid of the orange Telstra swirl on shutdown.
bootanimation part 1 = /sustem/media/bootsamsung.qmg <--telstra logo
bootanimation part 1 = /sustem/media/bootsamsungloop.qmg <--samsung logo
shutdown animation. = /sustem/media/video/shutdown/shutdown.qmg <--telstra logo
While I couldn't get rid of the Samsung logo at least I was able to get rid of the Telstra logo.
Also, you could try no boot or shutdown animation, just rename the above .qmg files to old.qmg
With no bootanimation its just a blank screen, and without a shutdown animation the desktop is displayed until the device turns off.
Hope that helps