Actually I've just used your example and worked out that the geotag.php file needs to be updated located in the syntax folder.
Line 72 and 73 need to be changed from
preg_match ( "(lat[:|=]\d*\.\d*)", $tags, $lat );
preg_match ( "(lon[:|=]\d*\.\d*)", $tags, $lon );
to
preg_match ( "(lat[:|=]-?\d*\.\d*)", $tags, $lat );
preg_match ( "(lon[:|=]-?\d*\.\d*)", $tags, $lon );
ref:
http://sourceforge.net/p/dokuwikispatial/issues/9/
It seems it's been updated, but not in this plugin.