it should work, it just will not be as efficient as a vertical one. water is constantly forced through the media over the entire time the media is in the vessel. gravity helps this in a vertical orientation. in a horizontal orientation as the media is used up it could settle leaving open water above the media. the water can pass through the reactor and not go through the media.
do not get me wrong the lower pH will still cause the media to desolve it just will not be as efficient.
my .02,
G~