You could refer to the DTD by means of a public id which is known to
the application. Other XML processors that don't understand that
public id will use the URL instead.
For example:
<!DOCTYPE mydoc PUBLIC "-//MyCompany//My product//EN"
"http://my.company.com/myproduct.dtd">
Your application would recognise the id "-//MyCompany//My product//EN"
and supply a built-in DTD instead of fetching the URL.
Perhaps someone else could give better guidance on just how to construct
a suitable public id.
-- Richard