You can specify a music on hold class with the M option in Meetme app. We could modify the page.conferences.php like this to acquire this feature:

<tr>
		<td><a href="#" class="info"><?php echo _("Music on Hold:")?><span><?php echo _("enable music on hold when the conference has a single caller")?></span></a></td>
		<td>
			<select name="opt#M" tabindex="<?php echo ++$tabindex;?>">
			<?php
				$tresults = music_list($amp_conf['ASTVARLIBDIR']."/mohmp3");
				if (isset($tresults[0])) {
					foreach ($tresults as $tresult) {
						echo '<option value="M('.$tresult.')"'.((strpos($options,$tresult) === false) ? ' ' : ' SELECTED').'>'.$tresult."</option>\n";
							}
						}
			?>		
			</select>		
		</td>
	</tr>

Replace this code to the table row that help you choose to enable Music On Hold. But we'll have to expand the option column in MySQL asterisk database, because now we use it to store the MOH category name as well as other options.
And of course, we can modify the meetme.conf to use a default MOH class.