MMBase

According to the editwizard documentation something like the following should work:

<lists>
<optionlist name="newslist"
optioncontent="field[@name='news.title']"
>
<query xpath="/*@mags/posrel/news" where="mags.number = 56" />
</optionlist>
</lists>

Only the query on all news nodes results in an optionlist in the wizards, for example:

<lists>
<optionlist name="newslist"
optioncontent="field[@name='news.title']"
>
<query xpath="/*@news" />
</optionlist>
</lists>