Well, I read the smarty documentation for the section function, and found that you can specify the max # of times it should step through the loop. Now, I don't have Smarty, so I can't test this, but I'm wondering if you can't do some arithmetic perhaps?
{section name="yearLoop" loop=$yearLoopEnd max=($yearLoopEnd - 1900) step=-1}
If not, you might have to either a) specify a number for max, or b) use a for() loop to manually output the years:
for($i = date('Y'); $i >= 1900; $i++)
echo "<option value=\"$i\">$i</option>\n";