As a test, I wrote a simple page that would leverage preview and execute across multiple modes in the same accelerator.

UPDATE: Thanks to some helpful folks, I have found that accelerator preview blocks the passing of documentUrl as a safety precaution for SELECTION accelerators and LINK accelerators.  So, the below XML should have those preview sections removed. documentUrl is passed for EXECUTE actions though.

   1: <?xml version="1.0" encoding="utf-8"?>
   2: <openServiceDescription xmlns="http://www.microsoft.com/schemas/openservicedescription/1.0">
   3:   <homepageUrl>http://localhost/IE8Demo/Default.ASPX</homepageUrl>
   4:   <display>
   5:     <description>Share web content with your IE8Demo friends.</description>
   6:     <name>Share on IE8Demo</name>
   7:     <!--<icon></icon>-->
   8:   </display>
   9:   <activity category="Query">
  10:     <activityAction context="selection">
  11:       <execute method="get" action="http://localhost/IE8Demo/AccelQuery.aspx">
  12:         <parameter name="src" value="selection"/>
  13:         <parameter name="u" value="{documentUrl}"/>
  14:         <parameter name="t" value="{selection}" />
  15:       </execute>
  16:       <preview action="http://localhost/IE8Demo/AccelQuery.aspx">
  17:         <parameter name="src" value="selection"/>
  18:         <parameter name="u" value="{documentUrl}"/>
  19:         <parameter name="t" value="{selection}" />
  20:       </preview>
  21:     </activityAction>
  22:     <activityAction context="document">
  23:       <execute method="get" action="http://localhost/IE8Demo/AccelQuery.aspx">
  24:         <parameter name="src" value="document"/>
  25:         <parameter name="u" value="{documentUrl}"/>
  26:         <parameter name="t" value="{documentTitle}" />
  27:       </execute>
  28:       <preview action="http://localhost/IE8Demo/AccelQuery.aspx" method="get">
  29:         <parameter name="src" value="document"/>
  30:         <parameter name="u" value="{documentUrl}"/>
  31:         <parameter name="t" value="{documentTitle}" />
  32:       </preview>
  33:     </activityAction>
  34:     <activityAction context="link">
  35:       <execute method="get" action="http://localhost/IE8Demo/AccelQuery.aspx">
  36:         <parameter name="src" value="link"/>
  37:         <parameter name="u" value="{documentUrl}"/>
  38:         <parameter name="t" value="{linkText}" />
  39:         <parameter name="l" value="{link}" />      
  40:       </execute>
  41:       <preview action="http://localhost/IE8Demo/AccelQuery.aspx" method="get">
  42:         <parameter name="src" value="link"/>
  43:         <parameter name="u" value="{documentUrl}"/>
  44:         <parameter name="t" value="{linkText}" />
  45:         <parameter name="l" value="{link}" />
  46:       </preview>
  47:     </activityAction>
  48:   </activity>
  49: </openServiceDescription>