Renaming role attribute for epub

["redlettucemail" <redlettucemail@mailscan.acenet.net.au>]

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