I think it would be much more efficient (i.e. catching all possible causes) and cheaper to simply install a fire detector. I.e. infrared sensors which immediately trigger alarm. And if one wants to do more, that could be combined with a sprinkler-like system (spraying up from below the bridge deck).
A fire detection system would be feasible. A laser beam/interuption detector would likely be suitable for both the Lobato and Cascade Trestles. Since there is no electric utility service available, some
research would be necessary to see if solar power would be adequate.
Notification might be a bit more complicated. It is unlikely that
cell phone communications could be provided. The existing VHF radio repeater system could probably be utilized for alarm communication, but the next issue would be the monitoring location and the availablility of firefighting assets to be dispatched.
Your suggestion of an automatic deluge type sprinkler system would require complicated and expensive assets. In the absence of both electric and water utilities at both trestles, the only alternative might be sprinkler systems supplied by gravity stand pipes. Additionally, freeze issues might limit use of such a system to operational season only.