I'm not sure whether this might work, but it's worth trying to see how it tastes..

(also not sure if the ingredients list in the product are suitable, but there's no beef or chicken flavours added to it, just a bunch of ingredients I can't pronounce...hence, might not be the very best substitute for chicken broth)

Could you substitue Carb Options Onion Soup Mix for the onions and the broth? The product has 2.5 grams carbohydrate per 5 gram serving "as sold" (meaning, without sour cream for dip or what have you). The box contains 2 envelopes of soup mix, and each envelope is 20 grams carbs.

Or maybe try a vegetable broth boullion type thing? Or even make your own vegetable stock in advance of making the pumpkin soup?

I'm reading the ingredients list on the side of the box of Carb Options onion soup mix, and I'm not clear on whether they'd be good for "fasting". Let me know if you want me to type out the ingredient list.

Happy soup making