The problem I could see with this setup is that you might lose some important context of the conversation. If the conversation initially takes place in DialogFlow and you get to a point where it should be handed off to Rasa (e.g. for checking the pending request), the information on what the user already provided has to be sent to Rasa as well (for example if the user has already mentioned the id of their travel request or their name, Rasa should not have to ask for it again).
As far as I’m aware you could use the HTTP API for this. See for example this thread going into that direction.
I don’t have any experience with implementing this though, someone else might have to jump in for further help…