choice of xsl modes in customization layer

Hello,

I have a docBook base that contains books and articles that are to be 
presented as HTML.

Seeing that there are docBook <article> and <book>, it would be 
desirable to make use of both, and to use chunking for the books (e.g. 
Table of contents, and single chapters each on another page), while 
articles do better displayed unchunked.

In the customization layer, I would think I'd have to build something like

if the dB starts with <article> --> use (i.e. import!) html/docbook.xsl
if the dB starts with <book> -->  use docbook-chunk

But I can't do that, since imports need to be top level, right?
So, is there a provision in docbook to get this done?

Thank you!
Nathalie