I've come across occasionally mentions of savoury meringues with flavours such as beetroot. How are these made? What is used as a substitute for the sugar?
The eggs whites in traditional meringues are used to spread the sugar into a thin foam that is then dried in the oven (or dehydrator) leaving behind the sugar structure and some proteins from the eggs. To make a meringue you need something that dissolves to tangle up with those proteins. I would guess that the beet meringues from Café Atlantico are made with beet powder replacing the sugar with the goat cheese in the middle adding to the sense of savory.
I have never made savory meringues, but if I were to experiment I would mix freshly whipped eggs whites with sugar to those created by reducing a savory liquid and then adding powder egg whites to it.
Well, you could always take a bite out of Adria's apple and just make foams directly from whatever liquid you wish to use (which may or may not be egg white; I'd advise against it. Why dilute your flavour?). Then add methylcellulose to provide you with the matrix you need for stability, and a standard ISI whipped cream dispenser will foam your product.
So for example, you could make a beetroot juice (250g) boiled with 50g sugar and 50g water, then cooled. Add methylcellulose 8g (2.2% by weight, using Methocel F50). Blend well. The recipe I have (for a carrot foam) calls for it to then be whipped in a stand mixer to stiff peaks, spread on a sheet and dehydrated for 5 hours. I imagine you could extrude from a standard ISI instead, probably charged twice with NO2.
Oh, as an added bonus, this would allow you to make completely vegan 'meringue' as well. Use a different liquid, add a touch of vanilla.
Sugar is crucial to making meringues!! It is what helps the chemical reaction with the egg whites and helps create that beautiful fluffy texture. However, I've come to own a fabulous book called "meringue magic" by Alisa Morov(who invented savoury meringues I believe). Amongst amazing savoury recipes she says you can reduce sugar to a certain amount but can never do without it. But the spices and salt make it a distinctively savory meringue.
1 Savory meringues are made with salt not sugar. They are primarily used to smother and braise a side of fish or chicken.  Cheesy Clouds This recipe can have added herbs and seasonings.