Thank you for your help, Jason! I did have to tweak it a little, but you nailed it!
function onPreCreate($Request)
{
if ($Request.Class -ne 'user') {return}
$UIDdecimal = $Request.get('uidNumber')
$UIDhex = [string]::Format("{0:X}",$UIDdecimal)
$Request.put('gecos',$UIDhex)
}