Is it ethically defensible to attain consent on an opt-out basis in a newspaper?
You'd need to obtain the first informed consent and examine the wording. If the informed consent does state that the collected samples or information may be used for other research-related purposes and the new projects will be announced on some specified channels, then the researchers have fulfilled their due diligence.

The problem being the general participants are really not very well-informed when it comes to providing biological specimen or medical records. For example, most of them may not realize that their samples can reveal a lot more as technology evolves. Unlike a questionnaire or an interview, the respondents know clearly what's written and said, but they may not realize what their blood or DNA can reveal, or what they can reveal a few more years down the road when molecular technology further develops, and the researchers want a re-analysis.

The researchers are usually not to blame, however, as they are probably equally clouded by the ever-changing hot topics. There are many occasions that a study was set up to test compound A, but fearing that compounds B, C, D... etc. may be the next scientific focal points, the researchers may opt for writing in the "opt out from reanalysis" statement into the consent form even they are not entirely sure what are to be reanalyzed and even if that analysis will ever happen. As for why opt out but not opt in, part of the reason is cost control: It's extremely difficult to track people down and collect a consent form again. For that, I can empathize.

If you will potentially be one of these studies' participants and are worried about this opt out mechanism, do realize that you don't owe them anything and you are completely free to walk away or require an amendment to limit the use of your samples, or request your sample or copies of record be destroyed after the study. The worst is that they will say no and you'll walk away without a chance to participate or receive the stipend. However, you'll have your peace of mind.

So to answer your question, this practice is defensible as other mechanisms can be cost-prohibitive. However, I personally do not feel it is entirely fair as I am doubtful that the participants fully know the consequences and their rights when it comes to giving biological samples or related medical records. Guidelines are being developed, and hopefully more education on how to be an informed participant will happen.