Found this thread while looking into implementing an alchemy/magic system of some kind.
Now, I'm a novice modder at best, so forgive me if I've misunderstood something, or made incorrect assumptions about something, but I think I have an idea that might work. It's only a theory though.
It seems to me, based on reading this, that the issue with applying a syndrome through inhalation is that due to the breathing tick, the target creature can only inhale a small number of times, as the gas fades before they have another chance to breath it in. Even when generating a large number of boiling stones, the gas still fades away in relatively the same amount of time, correct? And IndigoFenix's work around gives multiple chances for the effect to work, but runs multiple reactions.
My theory, I believe, may work relatively well. If you could extend the amount of time there's gas in the air, it would give the creature more chances to breathe it in and be affected. Containers affect the temperature of their contents, raising or lowering them depending on it's own temperature. What if you had the reaction produce a number of stones and place them in a container with a fixed temperature just over their boiling point? Give each stone a different spec_heat value calibrated so that when the gas of one dissipates, another one boils(would require testing to get the time right). Or better yet, have multiple clones that boil simultaneously so theirs a greater chance of the inhalation happening. This way, the air is full of syndrome-gas for a longer amount of time using one reaction, giving a more reliable(not 100%, unless you do it for a very long time, but close) rate of effectiveness.
I could be misunderstanding absolutely everything, but I'm pretty sure that'll boost effectiveness if it works.
Edit: May not be the most elegant of solutions...