I very much like the idea. Very flavorful and very genera appropriate. Thank you for bringing to my attention something I've been neglecting in my setting.
Some quick rule ideas off the top of my head, balancing the technique against more traditional potion making.
a) Potions meant to effect the imbiber made in smokable form take longer to have an effect, but have increased duration when they do so.
b) Any gaseous cloud spell can be made in smokable form and gains a free +1 caster level compared to bottled forms, but the pipe itself must either be in or adjacent to the area of effect.
c) Cure spells can't be made in smokable form, but other healing type magic (cure disease, restoration, etc.) still function.
d) Divination spells made in smokable form gain increased caster level compared to ones that are drunk.
e) Spells that induce sleep can be made in smokable form, regardless of their target or area of effect, and gains +1 caster level. However, range is limited in the same way as gaseous spells.
Characters which can Brew Potions can make the smokable version of the potion with a successful Craft (Apothecary) check at DC 15 + spell level. Failure ruins the potion. Alternately, at the time Brew Potion is taken, the character can indicate that they wish to specialize in smoked potions. In this case, they can make smokable potions normally but require a Craft (Alchemy) check to brew the imbibed form.
I'm going to need to create a range of exotic drugs as well. Of course, knowing the mentality of my current players, this addition to the setting is likely to provoke a lot of junior high 'teehee' behavior.
Oh well.