I would suggest to use "goog:chromeOptions" instead of "chromeOptions" and add "excludeSwitches": ["enable-automation"] in the options.
Since I don't have a crx flle, I tested loading extension folder and it worked OK, even though a balloon popped up warning not to use an extension when Chrome is in developer mode. My test capabilities are as below:
$sDesiredCapabilities = '{"capabilities": {"alwaysMatch": {"goog:chromeOptions": {"w3c": true, "excludeSwitches": ["enable-automation"], "useAutomationExtension": false, ' & _
'"args": ["start-maximized", "load-extension=C:\\users\\cycho\\appdata\\local\\google\\chrome\\user data\\default\\extensions\\pkedcjkdefgpdelpbcmbmeomcjbeemfm\\7819.902.0.1_0"] }}}}'
The extension folder is the folder where manifest.json file for the extension is located.