В общем, получил желаемое:
<?php
$dtfrom = JFactory::getApplication()->input->getString('dtfrom', '');
$dtto = JFactory::getApplication()->input->getString('dtto', '');
$db =JFactory::getDBO();
$query = 'select min(odate), max(odate) from #__mfo_docums where (client = '.$db->quote($ulogin).') and (loan = '.$db->quote($loan).')';
$item_minmax = $db->setQuery($query)->loadRow();
$dtmin = $item_minmax[0];
$dtmax = $item_minmax[1];
?>
<form action="<?php echo JRoute::_('index.php'); ?>" method="get" name="adminForm" id="adminForm" class="form-horizontal">
<div>
<input type="hidden" name="dtmin" id="dtmin" value="<?php echo $dtfrom; ?>" />
<input type="hidden" name="dtmax" id="dtmax" value="<?php echo $dtto; ?>" />
</div>
<div class="control-group">
<div class="controls">
<script>jQuery(function(){
var form = document.getElementById('adminForm');
jQuery("#dtfrom").datepicker({dateFormat:"dd.mm.yy",changeMonth:true,changeYear:true,showButtonPanel:true,minDate:"<?php echo JHtml::_('date', $dtmin, 'd.m.Y'); ?>",maxDate:"<?php echo JHtml::_('date', $dtmax, 'd.m.Y'); ?>"});
jQuery("#dtfrom").datepicker(jQuery.datepicker.regional["ru"]);
if (jQuery("#dtmin").val() == '')
{jQuery("#dtfrom").datepicker("setDate","<?php echo JHtml::_('date', $dtmin, 'd.m.Y'); ?>");} else
{jQuery("#dtfrom").datepicker("setDate",jQuery("#dtmin").val());}
jQuery("#dtto").datepicker({dateFormat:"dd.mm.yy",changeMonth:true,changeYear:true,showButtonPanel:true,minDate:"<?php echo JHtml::_('date', $dtmin, 'd.m.Y'); ?>",maxDate:"<?php echo JHtml::_('date', $dtmax, 'd.m.Y'); ?>"});
jQuery("#dtto").datepicker(jQuery.datepicker.regional["ru"]);
if (jQuery("#dtmax").val() == '')
{jQuery("#dtto").datepicker("setDate","<?php echo JHtml::_('date', $dtmax, 'd.m.Y'); ?>");} else
{jQuery("#dtto").datepicker("setDate",jQuery("#dtmax").val());}
});</script>
<div class="dt-filter">
<span><?php echo JText::_('COM_MFO_FILTER_DT_1'); ?></span><input type="text" class="datepicker" name="dtfrom" id="dtfrom" readonly>
<span><?php echo JText::_('COM_MFO_FILTER_DT_2'); ?></span><input type="text" class="datepicker" name="dtto" id="dtto" readonly>
<span> <a href="javascript:form.submit()">Apply filter</a></span>
</div>
</div>
</div>
</form>
Поля и при сабмите и при первоначальном открытии вьюхи отображают нужные даты.