Found: the trick is to put another table inside a cell of the table with the non-rowspan content, remove the rowspan from the other cell and set the outer and inner table height to 100%. Horrible but effective.
HTML:
<table id="outer"><tbody><tr>
<td>
<table id="inner"><tbody>
<tr id="row-1">
<td>1st row</td>
</tr>
<tr id="row-2">
<td>2nd row</td>
</tr>
</tbody></table>
</td>
<td>
a<br />
a<br />
a<br />
a<br />
a<br />
a<br />
a<br />
a<br />
a<br />
</td>
</tr></tbody></table>
CSS:
#row-2
{
height: 1px;
}
#outer, #inner
{
height:100%;
}
http://jsfiddle.net/xp7vz/3/