Is it possible to use Xojo Cloud to hold the web app and when data is entered by the user it is only transferred to an in-house database so no actual survey responses are kept on Xojo Cloud?
TIA
Yes I did this during the beta. You do need to open your in-house database to the Internet, but you have a static IP address for your cloud server and can limit access to this ip address in your firewall.
having this connection could have security issues. Make sure you sit down with security and tighten it up as much as you can so no third parties get access to your database. also I would have a way for your app to hold the data short term in the cloud in case you loose access to your database. There are lots of attacks (like DDoSes) out there that might affect something in the middle (not you per se but something you have to go through).
You also might consider having your office do a pull request once every 5 minutes using HandleSpecialURL mechanism. That way, surveys would only stay on the Xojo Cloud server for a short period of time.