The Hobie is designed for the rig to be tightened via the jib halyard. when the jib wire is properly tensioned, the forstay has about 12" (or so) of slack in it. Tightning the shrouds with the sail down will change the rig so that the mast rake is wrong when the main is hoisted. Using the main halyard as a backstay will rake the mast aft and further "loosen" the shrouds since they're aft of the mast step. That would, however, put some downward pressure on the mast - so it is an option. Best way to tighten the rig for mooring is to use the jib halyard - perhaps in conjunction with a "pigtail" to substitute as the jib luff - wire or low-stretch line. That will tighten both the forestay and shrouds.
Ultimately, this boat should be beached.