[xsl] Pushing the liquidity of templates?

Let's save I have a template for an an address form which I call using
apply-templates mode="address" select="/"> and I pass it existing
address data which autofills text input values.

I can use the select statement to match either the XML being
transformed or a remote variable, and this is great. But let's say the
mailing address information is coming from the XML being transformed
and the telephone and fax information is coming from a variable.

I know I could just use with-param but I'm wondering if there is a way
to pass information implicitly ( match="/" ) but also pass explicitly
when the "/" doesn't contain the neccesary data.

B) A series of when/choose, testing whether //phone exists and
declaring it if it is empty.