It is quite hard to get into other houses. Hôtel Solvay (a house, not a hotel) is near the Horta Museum and I believe it opens on request. But there are other properties you can see without too much trouble: the Musical Instruments Museum is in a former department store, Old England, and is worth seeing as a museum anyway. You can travel to the roof, eat in the Belgian restaurant there and see the view, as well as appreciate the wrought iron on each floor. They have done a really good job of preserving its features. The Comics Art Museum is also in an Art Nouveau building, a former fabric warehouse, also Horta. It is less spectacular than the MIM, but if you want to learn a little bit about the Belgian passion of cartooning, it's up your alley. You probably won't know any of the characters except the Smurfs. Finally, there is the Square Ambiorix neighbourhood, near Schuman metro. You can't go in any of the houses but it's a fairly well preserved Art Nouveau neighbourhood. Hôtel van Eetvelde and Maison Cauchie are two of the important houses there. Large parts of St.-Gilles and Schaerbeek are Art Nouveau, but you realistically probably won't get to see them unless you do a tour with ARAU. You would have to be lucky, though, as ARAU gives tours infrequently (they are a residents' action group) and possibly your dates won't line up with theirs. There are also Art Nouveau guides to Brussels available. Do have a google on this topic, there is more than I can feasibly cover here, but I hope I have given you a few leads.
Lavandula