Pablo, I don't see how this is possible...the svytrk object has
cntid SREL -> cnt.id REQUIRED
which means anonymous is not allowed. Besides, the web operation DO_SURVEY requires a valid CNT_ID value.
On the other hand, you may "workaround" this by allowing normal survey and then use pdm_load to extract all survey responses from Survey_Tracking for this survey and then modify the cntid field to the "anonymous" user id and pdm_load back to change the responses to "anonymous".
Thanks _Chi