So I just accidentally discovered something out of boredom - save yourself a bunch of clicking if you use WinRAR on Windoze:
"%programfiles%\WinRAR\winrar" a -ep1 -ibck -r "%appdata%\.minecraft\bin\minecraft.jar" .\skin\*.*
To use, assuming that WinRAR is installed in your "Program Files\WinRAR\" dir (for most, "C:\Program Files\WinRAR\"), if not, adjust to point to it
And put your texture pack in a folder in the same folder as the bat called "skin" (or change it to point to the right folder)
Then just run the bat
So you should have like:
combine.bat
skin\
terrain.png
etc
Combines successfully for me. Even if it doesn't work for others who have things installed in weird places or don't use the same programs, it's still a handy thing!