So my question is this: Is what I want to do feasable as a way to get a static address and act as an AMPR gateway?

Yes, this works quite well. I have the same setup here. I have a colocated Raspberry Pi that is on the IPIP mesh using ampr-ripd in the standard setup that is described on the WiKi, and it serves my subnet and a couple of /32 addresses.

My subnet is tunneled to my home using IPsec in AH/Tunnel mode. Using AH instead of ESP (as is usually used) reduces CPU usage, and encryption is not required for this purpose anyway. At home I have a MikroTik router.

I do have a fixed IP at home so IPsec is easy, when you don't have that it may be easier to use a VPN that does not care about addresses, e.g. OpenVPN.

I also run a couple of applications locally on the Pi, hence the /32 addresses.

Today, thanks to Marius' work, it is also possible to use a MikroTik router instead of the Raspberry Pi. It may be easier to configure and is available in rackmount formfactor.

Rob