XSLT document()

I have a need to process 2 XML instances in a single piece of XSLT (I'm 
doing some x doc validation).

I see that in XSLT I can use the document() function BUT it APPEARS that it 
must retrieve the XML from the file system or a resolvable URI ??

In my case the second instance doc is already 'in memory' (having been 
retrieved from a database) so I really just want to pass the XML itself as a 
string.

Also, because the production environment in which the solution will be 
deployed is (naturally) constrained, getting access to the file system to 
create a temporary file is unlikely to be possible (plus performance could 
be an issue for some of the quite large XML docs we use).

Does anyone know whether this is possible using XSLT, or will I need to look 
at a programmatic (ie. Java) implementation alternative :-(

Fraser.