Renaming role attribute for epub

For epub 
transforming, there is coding in epub docbook.xsl for book cover images 
of:

d:imageobject[@role='front-large']
 

I would 
like to use an attribute role='epub' in my markup. If I change the 
imageobject role value to 'epub', how do I then remap that back to 
'front-large' so the stylesheet works?

Thanks,

Dave Gardiner