Schema-aware pretty-printing

Hi,

I have a some very large XML source files.  Each is are currently formatted
as one long string (ie no line breaks).

I need to add some processing instructions at various points in the file but
I don't want to introduce any significant whitespace in the process of doing
so.

I have XMLSpy but it does not seem to be capable of doing a schema-aware
"pretty-print". 

A web search seems to indicate that OXygen is capable of doing an
intelligent pretty-print (the information I found is in the context of XHTML
but I assume it's more generalized than that??), however, I can't really
justify buying it just for that. 

If needs be, I can write an XSLT to parse a DTD, classify each element as
"inline", "mixed", "block" or "empty" and then do an identity transform
against my XML source files inserting line-breaks and leading tabbing as
required.

I am wondering if anyone else has any other suggestions for me?

Thanks,

Murray