Hi Jen, great question.
The mucilage in marshmallow is best extracted in a cold water infusion (so, you will get the most mucilage in water), but a tincture with a solvent of water and alcohol still provides great medicinal value. I have personally used a tincture of the root many times (which is very thick and viscous), for myself and clients who are not tea drinkers, and it has worked wonders. Mucilage is precipitated out if combined with strongly tannin-tasting herbs, thus becoming less bio-available, so this is good to keep in mind if you’re combining a blend with other herbs.
You could make a cold water infusion overnight, and then add a bit of alcohol for preservation if you’d like.
Let me know if you have more questions!
Riley