While I'll happily admit that my opinion might be wrong, I don't think the situation is "exactly" like the wizard situation. The wizard treats his longsword as though it were a wand, yes, but "wand" is not a weapon property; it's an implement. So Wizards treating their swords as if they were wands would be the same as the multi-class ranger treating his sword as if it were a dagger (or handaxe or whatever) and taking all the hits to damage dice and the like. But that's not what the feat says.
In D&D4, 'wield/use foo as though it were/instead of/as a [/i]bar' has a specific meaning: That anything that benefits/hinders bar is applied to foo. So yes, it is -exactly- the same.
'Use sword as though it were wand' means stuff benefitting wand benefits sword. It has nothing to do with weapon types or implement types, but the general rules template.
Is weapon in your off-hand? Check. Is it one-handed? Check. So you treat it as though it were an off-hand weapon. That means not only is this configuration permissible, you get the damage bonus from tempest. Otherwise, you are -not- treating it as though it were an off-hand weapon, and you are not obeying the pertinent rule.
Contrast with 'You may wield one-handed weapons without the off-hand property in your off-hand.' That is a different rule, and applies different baggage.
The rule is granting an exception to a general rule, but it's not the exception you think. It's not saying 'You can wield non-off-hand weapons in your off hand.' it's saying 'The benefits of the off-hand property are applied to all one-handed weapons in your off-hand' which is absolutely different.
And as well, it doesn't do anything for a weapon in your main-hand, so if you want the damage bonus, make that an off-hand weapon. Double-weapons are -still- the place to go here.