disclaimer: I don't use android

Having said that I hack on android init systems a bit.

What you say will (should) work just fine.
I assume you mean mount /datatemp as /data on reboot - otherwise you'll find you can't unmount it if it's in use.
If you don't mean on reboot then you may be able to mount the sd card on top of /data and 'hide' the 1Gb partition


For the reboot approach: Android is (partially) linux, linux uses fstab files to map devices to mountpoints. Android has fstab files - you'd think...

However some vendors will hardcode mounts into the init.* scripts in root and you'll need to find how this is done on your device and change it. Basically "grep mmc /*" in the / dir (Android isn't known for being tidy with config files either)

Make sure you have a reliable way to get back into the device before hacking on any of this as it's possible to break booting.