Half a pound sounds about right for 5 gallons. Maltodextrin can be used just like lactose; both are non-fermentable.
But maltodextrin gives a more malty profile and not as much overt sweetness, lactose gives a milky sweetness.
If you're worried about it, lactose in beer might cause troubles for people who are extremely sensitive to lactose. Maltodextrin just gives you beer farts (the bugs in your gut have a fiesta when they encounter it and produce lots of methane by way of thanks).
A third option is to use an artificial sweetener, like saccharine or sucralose. But, if you go with any of these sweeteners, you need to be really careful. In beer they can sometimes seem unpleasantly bitter or obviously artificial.
Another option, if you're kegging a beer for a one-off event, is add a bit of fermentable sweetener to the keg, keg just a day or two before the event and keep your keg cold until the party. That way, even if you do get secondary fermentation, it won't have time to go to completion and you can bleed off any CO2 that develops. That's a nice way to handle relatively delicate sugars, like subtly-flavored honey.
Personally, I'd use lactose, but I'd draw a beer sample and test adding various amounts of lactose to get the sweetness right. There might be enough residual sweetness in your beer that you don't want to overdo the sugar addition.