Looking for advice on empty tag usage

Hello,

I'm working on creating an XML schema to be used with XSLT and Web
Services that interfaces with a legacy system.  On this legacy system
there are fields that accept values like "*" to signify that the user
wants to clear whatever was stored in that field before.   I would like
to create something more user friendly and intuitive (while still
allowing a user to send an asterisk to mean clear as well) and so some
ideas I have come up with are:

1) Use an empty tag such as <ClearNarrative/> and then putting a group
choice of <ClearNarrative/> and <Narrative/> within my sequenced type.

2) Use an attribute to signify that the field should be cleared on the
legacy system.  Only problem is I don't see a way to use the attribute
with just listing the attribute without a value (i.e. <Narrative clear>
instead of <Narrative clear="Y">.

I cannot just assume that if <Narrative/> is sent to my Web Service that
the user wants the field cleared as some programs that interface may
decide to send all tags whether information is being passed or not.

I was wondering if there might be a best practice around on how best to
solve my problem.  Or if anyone might have some wisdom to share on this
type of situation.

Thank you in advance,
Rebecca Wilde