We have had the following situation in our new engineering building labs. Our state's building code
BOCA international, required "ducts" to be sprinklered the same as the rest of the building. This provision was enforced by  TAHJ.  To meet this, we added an BC suppression system, much like a cooking dry chemical suppression system, which satisfied TAHJ.  When the acceptance inspector arrived, there were two jars of metals in storage adjacent to the hood, when I asked him if they reacted in the hood, what effect would the suppression system have on them, he said (correctly) not too much if any.

We petitioned and won an exemption for suppression in hoods and fume hood ducts based on computer modeling and info from MSDSs on the materials most likely to be used in the lab.

We are continuing to sprinklering all labs along with the entire building in all new construction
and considering Class D portable extinguishers
for those few areas with a metal problem.