White lacecaps are not the easiest thing to find! My favorite is a macrophylla called 'Queen's Lace' that offers very large lacecap flowerheads, but it's rather uncommon. And it will require winter protection in your zone, which may not be the most attractive look for your front yard. You could try H. paniculata 'Chantilly Lace' - a very hardy but smaller growing hydrangea that has lacecap-type flowers that start out white but fade to pink and green. Or a relatively new introduction of arborescens called 'White Dome' that looks particularly good in photos. Have not seen one in the flesh to judge personally. Both of these should be quite hardy for you without protection.

Figures I'd get an idea in my head for something that doesn't quite exist! White Dome looks promising - I hope it will do ok in semi-shade. Thanks for the ideas!

While 'White Dome' might formally fits the lacecap image you have in mind, outer sterile florets are small and not produced in abundance, so you might be somewhat disappointed with your choice. This shrub rather belong to woodland border/natural plantings than to the front of the house.
That said, I have it for two years only and it might be different at maturity, but I guess when shrub is already 3x3 and had a more than dozens of blooms, but still barely noticeable it doesn't portend well for its wide usage.
Alternatively, for the similar, but more prominent 'white effect' you may want to use native H. arborescens species, NOT a widely available 'Annabelle'.
It will produce small white mophead-type flowers in loose clusters, so overall effect will somewhat resemble lacecaps.
Best of all, it doesn't need any protection in your zone, though I don't see it offered for sale very often here.

Here is a link that might be useful: H. arborescens species

Irene,
Check out Coerulea Lace (link below). Should handle a good bit of shade and is supposedly very cold hardy to zone 5. Maybe if the location is a very well protected one, (south sounds good) you could get blooms without extraordinary winter protection methods. Never know unless you try, but if it works for you, please drop a note to let everyone know. I'm sure a lot of others are looking for the same thing.....yg

Here is a link that might be useful: Coerulea Lace / Hydrangea Plus