[assassingod]

You could do this by template modifications. Open up the priv_sendprivmsg template, find:

Please complete the recipient, subject and message fields.

replace with

Please complete the recipient and message fields.

then find:

	<td bgcolor="#DFDFDF"><normalfont><input type="text" class="bginput" name="title" value="$subject" size="40" maxlength="85" tabindex="2"></normalfont></td>

replace with

	<td bgcolor="#DFDFDF"><normalfont><input type="text" class="bginput" name="title" value="(no subject)$subject" size="40" maxlength="85" tabindex="2"></normalfont></td>

Untested, but should work