The Defense boost from Skull Bash is only temporary, given after the, " lowered its head!" prompt, and after the, "" used Skull Bash!" prompt.
Because Power Herb would nullify the first turn of charge, there is no trigger to give the boost, so my best guess would be no. :(
EDIT: The Defense boost was only temporary from Gen I to III, then was made permanent in Gen IV. Even still, I don't think this boost would happen.