FO invisible data

Greetings xsl-list

I am trying to extract some data from a proprietary medical data system. The
system basically generates a FO file that from the system goes on to be
printed/faxed/emailed to many sources. I can grad the FO file on its way
past copy it and do some other processing that I need to do to it. 

I have access to the XSL file that did generate the FO file and I can modify
it as long as the FO file appears the same when printed/faxed/emailed. 

However the problem I have is that there is one key piece of data missing
form the FO file that I need. I can insert it into the XSL file quite easily
but when I do it appears on the final output and I can't have that.

So my question is: Is there some way to include something like below but for
it to have a property perhaps that makes the formatter ignore it completely
but will still allow me to get to the data in the FO file?

Eg: 

Currently I have things like this
<fo:table-cell>
<fo:block xsl:use-attribute-sets="ItemHeading"
font-weight="bold">MRN:</fo:block>
</fo:table-cell>
<fo:table-cell>
<fo:block xsl:use-attribute-sets="ItemData"><xsl:value-of
select="/Report/Body/zSpRptSepSumPatDemogSP/MRN"/>
</fo:block>
</fo:table-cell>
 

But I want the same thing but to be not visible on the page. 

Is it possible to put in an invisible table or block? 

Thanks for any help out there

Alan

Alan Boldock
Senior Analyst Programmer 
Information Technology Services
Margaret Graham Building Level 2
Royal Adelaide Hospital 
South Australia
Speed Dial: 1953
Phone: 0411 659 867
Email: aboldock@xxxxxxxxxxxxxxxxxx