There are two more references to $this->modx->resource in the Wayfinder class -- lines 170 and 171. I'm not sure if fixing them will help, but it can't hurt.
Another thing you might try is leaving them as they were, and doing this in your plugin:
$modx->resource =& $resource;
Also, if you add an 'id' field to the array you send to Wayfinder, WF will use that for the 'here' location.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html