There may be a way. For the Galileo release, we have introduced a new
feature called "Design-Time Skinning". There is currently no documentation
and the feature's classes are not yet considered API (they are, therefore,
subject to change in subsequent releases), so the best source of
information at present would be to look at the implementation in the
org.eclipse.jst.jsf.apache.trinidad.tagsupport plug-in, which you can
access via CVS or which you can browse source of if you download the SDK
version of WTP.