Chances are as a newbie you can not participate in signature bounty campaigns. This is because there is no space for signature link since you have not been upgraded. In most cases for you to be accepted, you must be a Jr member. You to be active and must have done up to 30 activities to be ranked so. As a newbie your best bet is to participate in other bounties like facebook and twitter campaigns.
you are right. rarely seen signature campaigns with newbies allowed. but it is not so difficult to get that rank, takes about 1.5 month as i remember. while that - you can participate other tasks.