For standard image recognition CAPTCHAs you can send the image to the user and ask for a text answer.
With mCaptcha and other proof-of-work captchas it’s more difficult, but you can send a webxdc to the user that computes a proof-of-work (very cursed idea, please don’t, there are many better things to do with .xdc) This can indeed be integrated into DeltaChat in the form of Hashcash, but I am not sure it deters the bots, it’s mostly a tool against spam, unless you want to force computing it for every message and drain the user phone battery.