Is it possible to make a marshmallow that isn't sweet?

What about using mochi, an Asian pounded rice paste? It's a similar—though not identical—texture, it's available in sweet and savory forms, and it's held together by the starch in rice rather than anything gelatin, so it's vegetarian. You can get plain unflavored mochi at many Asian food stores; it may take some looking, as it's more often sold sweet and filled with bean paste in the US. I've had it in savory Korean dishes and I believe it's used in savory Korean dishes as well.

I've used agar-agar recently, and I think it might do the trick for you. A recipe plus some technique discussion is behind this link.

In the comments, a reply has been posted that the marshmellow would probably ´taste of the sea´ as agar-agar is seaweed based. This has not been my experience. However, paying homage to its heritage by using some sea-salt seems appropriate.

I'm a bit troubled by too many associations with the sea due to your use of mushroom paste. Selecting a flavor feels a delicate matter. As I'm getting a Japanese vibe from this dish, I would probably investigate shiitake as a mushroom. (If you weren't emulating chocolate, I'd suggest enoki as well).

I am highly tempted to investigate edible seaweed flowers (the existence of which I'm totally unsure about and just dreamed up - at least this link seems to support their existence). If their taste matches your needs, great. And, presuming they look nice as well, they'd be an interesting edible garnish on your plate.

Update: and for additional savory tastes, I'd look into other savory flavour often associated with seaweed or shiitake in oriental dishes - soy, ginger, cilantro, etc.

Originally marshmallows were made from the root of the Marsh Mallow (Althea officinalis). I have not yet tried making marshmallows this way, so I do not know if using mallow root would give you the exact same texture as our modern marshmallows. You may be able to find marshmallow root at a local health store, or there are plenty of sites that carry it online.

I was able to find one marshmallow recipe using powdered marshmallow root, about halfway down the page at http://www.hungrybrowser.com/phaedrus/m010702.htm. This too is a sweet recipe, but perhaps you could substitute your chosen savory flavorings in place of the sugar and vanilla.

I also found a single Marshmallow recipe claiming to be savory: http://www.popsci.com/diy/article/2008-11/anatomy-marshmallow?page=1

This is super old so I'm sure you've moved on but I felt compelled to comment. The Mission Street Food cookbook has a recipe for "mozzarella mousse" in which they put fresh mozzarella through a whip cream dispenser. I bet that would be a pretty good savory imitation of marshmallows.

http://highered.mcgraw-hill.com/sites/0078616441/student_view0/food_science_activities/demystifying_meringue.html

The problem is you need the sugar for the structure of the merringue. You might consider a sweet and sour or sweet and salty flavor profile for the dish so the merringue adds the sweetness. You may also be able to use some flavored vinegars (not sure what this will do to the structure) to cut the sweetness. Oh, I just noticed when this was posted. What did you finally do?

use isomalt to replace some of the sugar since it's much less sweet, but still has the properties of sugar

Ok, I know this thread is really old, but why couldn't you use egg whites beaten with lemon juice or cream of tartar for added stability, and then incorporate the dissolved gelatin into the meringue? You'd have to use less water, obviously. Or could you perhaps thicken milk or broth with corn starch to sort of mimic the consistency of sugar syrup? I guess it'd be like mixing in a thin gravy to which gelatin had been added. I'm intrigued. The only other option I've found was to use a balsamic vinegar syrup in place of the sugar syrup, but obviously that would still add sweetness.

Depending on availability, you might find an interesting substitute using puffball mushroom. The mushroom is white, kinda fluffy, and savory, and can easily be large enough to cut nice marshmallow-ish squares.

The fresh mushroom may be difficult to source, though it would be a nice option if season and availability permit. Dried, on the other hand, is easier to aquire, and very light - which might make a good mushroom marshmallow texture. The dried puffball could be re-hydrated or probably even eaten as-is if the texture is preferable (I have found it to have a very light and fluffy texture and I think it could be eaten plain, in the proper context - though the flavor is quite mild).

Supposedly the taste if prepared well is mushroomy and cheesy (which bodes well for pairing with mushroom paste for the 'chocolate'). Puffballs, according to the article I found, need browning to unlock the best flavor... and having a white cube with the outer edges toasted to golden brown would not be a bad thing on something imitating smores-style marshmallows.

On a completely different note, the article also mentions that if the puffballs are boiled (well, heated in water) they turn into something "a bit like mushroom marshmallows" texturally speaking. It may be worth it to poach or steam the puffball cubes (post-browning, for extra flavor) to get the texture a bit more marshmallowy.