I dont know but saw a thread about this recently that said they dont work on layed flat, like I said I have no idea as never tried but just wanted to let you know there is another opinion...
Water enters the vessel at the top, travels down through the resin then enters the bottom of the 'riser' tube in the base of the vessel. If there is air in the vessel when you lay it down then only half will escape as the riser opening is at the centre of the vessel so as the vessel first fills, water will drive the air out until the riser opening is below water level then the remaining air can't escape. This means the resin in the upper half of the vessel is not being used. Turning the vessel will put this resin into the water, but it will be difficult to know if all the resin is depleted.
It may be possible to fill the vessel first, but you must take care not to allow any water to escape and be replaced with air before you connect it up. Also if there is any air in the rest of the system (Prefilter housings, etc) this will travel to the vessel and then get trapped.
Much better to use the vessel upright, as it is intended to be used, or buy a smaller vessel.