Query String Keeper plugin for YOURLS - tested on version 1.7.

Query String Keeper retains the query string added to the short url and adds it to the long url. If the long URL already includes a query string, it appends it following an ampersand (&) and if not, appends it to the long url following a question mark (?).

Examples 1 & 2 use the following short & long URLs configured in YOURLS:
Short url = http://sho.rt/keyword
Long url = http://www.longurl.com

Example 1
http://sho.rt/keyword?foo=bar
...becomes...
http://www.longurl.com/?foo=bar

Example 2
http://sho.rt/keyword?123
...becomes...
http://www.longurl.com/?123

Example 3 uses the following short & long URLs configured in YOURLS:
Short url = http://sho.rt/keyword
Long url = http://www.longurl.com?foo=bar
Example 3
http://sho.rt/keyword?zoo=car
...becomes...
http://www.longurl.com/?foo=bar&zoo=car

1. Move query-string-keeper directory to the /user/plugins directory of the YOURLS installation.
2. Go to the Plugins administration page ( eg http://sho.rt/admin/plugins.php ) and activate the Query String Keeper plugin.

This plugin adapted from the Query String Forward plugin by llonchj which doesn't handle passing a single number as in sho.rt/foo?123.
The Append Query String plugin by drockney works for those that want the query string to just be appended without the ? or & symbols.