I just wanted to point out the renderas ("render as") attribute, which actually might be closer to what you're looking for. This would let you use section levels correctly, but you could select individual ones to be formatted differently, e.g.
<sect1>
<sect2 renderas="sect3">
....
<sect2>
...
See
http://www.sagehill.net/docbookxsl/Typography.html#UsingRenderas---
David Goss, M.A.
Technical Writer, Laboratory Division
Frontier Science & Technology Research Foundation
4033 Maple Road
Amherst, NY 14226
(716) 834-0900 x7218