Hi,
yes you're right it is not currently possible to set this.
I'm pretty sure the reason is that nobody ever needed it, and we didn't want to flood the configuration with all lucene settings.
I've taken a look at the code and it is very simple to add, I'll be glad to add it but you have to convince Emmanuel (proj. leader) that it is important to have this setting configurable.
Open a feature request on Jira:
http://opensource.atlassian.com/projects/hibernate/browse/HSEARCH
and please write there why you think this should be needed.
(I have played myself with this setting a bit but must recognize I didn't fully understand it's potential.)