Can I use liquid-form sugar to replace powdered-form sugar in meringue or sponge cake?
In theory you should be able to replace the sugar with a liquid form. The purpose of sugar in meringue is to stabilize the foam generated by the whipping. It does this by preventing the foam from drying out and collapsing as the structure becomes weaker from a lack of water. Hence anything that can retain the water will help.
The one major difficulty that you might encounter is that liquid sugars contain water already, so they might make the foam too liquid/soft to retain the meringue texture. You may be able to overcome this by whipping a lot more, but it might not work. To get around this you could substitute some of the sugar with guar gum or xanthan gum, at 0.04%, both of which appear to stabilize the foam formation (PDF; see page 88).