How do I pass a hidden input value to plugin onSaveUser event handler?
I found a solution. Though I am not sure if this is the best way to go about it, but here's what I did:

&lt;input type="hidden" name="userType" value="org"&gt;


was added to the register form, and ...

$userType = craft()-&gt;request-&gt;getPost('userType');

if ($userType == 'org')
{
    // ...


was added to the onSaveUser event handler function.