Attaching the population filter without using form IDs (suitable where the current form is on the same page as the URL in forms action parameter)

<?php
// Like so

$this->getContext()->getRequest()->setAttribute('populate', new AgaviParameterHolder(array(
    'question[0]' => 'Can you eat cheese?',
    'answer[0]' => 'No'
)), 'org.agavi.filter.FormPopulationFilter');

// Or like so...

$populate =& $this->getContext()->getRequest()->getAttribute('populate', 'org.agavi.filter.FormPopulationFilter');
$populate = new AgaviParameterHolder(array(
    'question[0]' => 'Can you eat cheese?',
    'answer[0]' => 'No'
));

Use form ids to link the pre-population to a particular form

<?php
// With ID

$populate =& $this->getContext()->getRequest()->getAttribute('populate', 'org.agavi.filter.FormPopulationFilter');
$populate['form-id-1'] = new AgaviParameterHolder(array(
    'question[0]' => 'Can you eat cheese?',
    'answer[0]' => 'No'
));
$populate['form-id-2'] = new AgaviParameterHolder(array(
    'question[0]' => "How many frags do I have to get before I'm considered awesome?",
    'answer[0]' => 'At least 15'
));

The above hints were created with help from IRC.