A method for setting a packer within a wellbore with a lower end defining an open-hole portion, comprising: providing a packer, the packer comprising: an inner mandrel, a movable piston housing retained around the inner mandrel; alternate flow channels along the inner mandrel, one or more flow ports providing fluid communication between the alternate flow channels and a pressure bearing surface of the movable piston housing and a sealing element external to the inner mandrel; connecting the packer to a tubular body, wherein the tubular body is (i) a sand screen comprising a base pipe, alternate flow channels, and a surrounding filter medium, or (ii) a blank pipe comprising alternate flow channels; and running the packer and connected tubular body into the wellbore, wherein running the packer into the wellbore comprises running the packer into the open-hole portion of the wellbore; running a setting tool into the inner mandrel of the packer; pulling the setting tool to mechanically shift a release sleeve from a retained position along the inner mandrel of the packer, thereby releasing the piston housing for axial movement along the inner mandrel; and communicating hydrostatic pressure to the piston housing through the one or more flow ports, thereby axially moving the released piston housing along the inner mandrel and actuating the sealing element against the surrounding wellbore, wherein the method further comprises: injecting a gravel slurry into an annular region formed between the tubular body and the surrounding open-hole portion of the wellbore, and further injecting the gravel slurry through the alternate flow channels to allow the gravel slurry to bypass the sealing element so that the open-hole portion of the wellbore is gravel-packed below the packer after the packer has been set in the wellbore. providing a packer, the packer comprising: an inner mandrel, a movable piston housing retained around the inner mandrel; alternate flow channels along the inner mandrel, one or more flow ports providing fluid communication between the alternate flow channels and a pressure bearing surface of the movable piston housing and a sealing element external to the inner mandrel; an inner mandrel, a movable piston housing retained around the inner mandrel; alternate flow channels along the inner mandrel, one or more flow ports providing fluid communication between the alternate flow channels and a pressure bearing surface of the movable piston housing and a sealing element external to the inner mandrel; connecting the packer to a tubular body, wherein the tubular body is (i) a sand screen comprising a base pipe, alternate flow channels, and a surrounding filter medium, or (ii) a blank pipe comprising alternate flow channels; and running the packer and connected tubular body into the wellbore, wherein running the packer into the wellbore comprises running the packer into the open-hole portion of the wellbore; running a setting tool into the inner mandrel of the packer; pulling the setting tool to mechanically shift a release sleeve from a retained position along the inner mandrel of the packer, thereby releasing the piston housing for axial movement along the inner mandrel; and communicating hydrostatic pressure to the piston housing through the one or more flow ports, thereby axially moving the released piston housing along the inner mandrel and actuating the sealing element against the surrounding wellbore, wherein the method further comprises: injecting a gravel slurry into an annular region formed between the tubular body and the surrounding open-hole portion of the wellbore, and further injecting the gravel slurry through the alternate flow channels to allow the gravel slurry to bypass the sealing element so that the open-hole portion of the wellbore is gravel-packed below the packer after the packer has been set in the wellbore. injecting a gravel slurry into an annular region formed between the tubular body and the surrounding open-hole portion of the wellbore, and further injecting the gravel slurry through the alternate flow channels to allow the gravel slurry to bypass the sealing element so that the open-hole portion of the wellbore is gravel-packed below the packer after the packer has been set in the wellbore.