I am a blue water sailor. I, and many others like me, would be happy to carry an ADS-B reciever onboard. That is, provided that it draws very little power, and that it gathers data unattended without my active intervention. Statistically, I think cruising sailors would cover a large fraction of the ocean areas of the globe. I believe the probability of a sailing vessel being within 200 miles of MH370s final flight path would be almost 100%.
The caveat being that I can not transmit the data to the Internet until the next time I reach shore and I can find someone who will let me plug in a USB device. That could mean a delay of months up to a year.
Would non-real time information be valuable? Thinking of the MH370 case, the answer must be yes. Not matter what the delay, the information is still valuable to someone. We could also record AIS signals that many vessels already transmit. I receive AIS from up to 40 miles away.
The idea could be etended to (symbolic) notes-in-a-bottle. A million floating ADS-B recorders would eventually reach shore, and some of them may have their data extracted and transmitted, then thrown back into the sea. Would that be worthwhile? Hard to say.