Thanks @Fran_Vidicek for the shoutout! 
STP won’t really solve the frontend/backend separation unfortunately. But I built a few frontend/backend docs where the transport is
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Forms: 🍕 Viva Pizza: Making a secure storefront (pizza delivery) on Coda
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A pack with a formula that has a side effect (i.e. saves the data elsewhere but is not marked as Action so that it can run in Play Mode): me collecting emails on my landing page (see Coaching → The Courses)
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Demo now defunct but the principle remains: a hosted embed with a way to trigger a webhook: Itsy: A Free Pack for Writing Web Apps that Run in Coda - #11 by Paul_Danyliuk