Is this strategy for runtime fixed-width->xml conversi

I'm wondering about viable strategies for runtime conversion of
fixed-width messages into XML (and back, I suppose).  I know there are
various frameworks that do this, but it occurred to me this could be
done entirely with XSLT.

The stylesheet would have a single template element, which would cover
the entire message format (unless there are conditional pieces).  The
input to the stylesheet could be empty, and a single stylesheet
parameter would be the "input" to the stylesheet.  Each reference in the
template to the input would use the "substring()" function, to pull the
appropriate portion of the parameter into the element.

I'm not sure exactly how the reverse direction would work, however.

Is this approach at all worthwhile?  What are specific problems with
this idea?