Due to the dearth of power armour in 1/72 or 20 mm scale, I asked for help on a mailing list. Someone replied that GZG 25 mm power armour will work just as well. My first reaction was "but they're too big!" However, since I believe in empirical observation, :) I actually took out a couple of my GZG 25 mm NSL power armour and did some comparison shots. The results are rather surprising.
From left to right, Caesar 1/72 modern French, GZG 25 mm NSL power armour sergeant, Caesar 1/72 nun, and another Caesar 1/72 modern French.
From left to right, Caesar 1/72 modern French, GZG 25 mm NSL power armour, Caesar 1/72 nun, and GZG 15 mm alien Ixx.
From left to right, GZG 25 mm NSL power armour with plasma gun, GZG 15 mm alien Ixx, GZG 25 mm NSL power armour, and Caesar 1/72 nun.
From the pictures, it's pretty clear that the 25 mm GZG power armour can be used with 1/72 miniatures as some sort of large armoured suit. It surprised me quite a bit that it would fit so well. Oh well, I guess I have power armour miniatures for my sci-fi campaign now.