It would be useful to be able to set the security level on an issue to include everyone who participated on an issue so if you had a security level that was only the reporter and the assignee, if the issue needs to get reassigned the issue could still be seen by the original assignee. The JIRA Toolkit has a participants custom field which could be used to do this, but unfortunately it doesn't show up in the "User Custom Field Value" section.

However, I tried to modify /secure/admin/views/issuesecurity/addissuesecurity.jsp and hardcoded the participants custom field as an option for the "User Custom Field Value" section:
e.g. from JIRA 3.13.4

<webwork:elseIf test="types/(.)/type == 'userCF'">
       <td>
             <select name="<webwork:property value="."/>" onClick="document.forms['jiraform'].type[<webwork:property value="@status/index"/>].checked = true;">
                <option value=""><webwork:text name="'admin.schemes.issuesecurity.choose.a.custom.field'"/></option>
                <webwork:iterator value="types/(.)/displayFields" >
                       <option value="<webwork:property value="id"/>" <webwork:if test="../../parameter(..) == id" >selected</webwork:if>><webwork:property value="name"/></option>
                </webwork:iterator>
                <option value="customfield_10020">Participants</option>
             </select>
        </td>
 </webwork:elseIf>

This seemed to work so It would be good if we picked up any custom field that implements UserCFNotificationTypeAware (I'm guessing that is why this works).