populate nodelist variable with a variable in select

Hi,
I am trying to populate a variable with a nodelist that takes a variable in
its select statement.

For example,
<xsl:variable name="myNodeSet" select="site/assets/asset[(type/@code = 'aaa')
and starts-with(@vpath,'my/path/')  and (lang='us_en') and $SEARCH_STRING]"/>

and SEARCH_STRING is defined as
<xsl:param
name="SEARCH_STRING"> and ( (related/@code = '111') or
(related/@code = '222') or (related/@code = '333') )</xsl:param>

My xml is of the form,
<site>
<assets>
<asset vpath="my/path/and/more">
<lang>eng</lang>
<featured-item>No</featured-item>
<type code="aaa"/>
<related code="111"/>
<related code="777"/>
</asset>
<asset>
  ...
</asset>
</assets>
<site>

Is
this even possible? I can do this using <xsl choose> and checking
each condition and setting the variable accordingly, but in future if i
have more conditions for the variable, i need to add more conditions in
all my xsls. It would be nice if i can set this variable in one place
and then use it use it as a replace string.

Thanks for any help or pointers in this regard.