In theory you should be able to install alternative operating systems by disabling "secure boot", but at least with Linux that isn't working properly yet. Trying to use the NVMe controller results in reboots, caused by the T2 chip. It's not clear so far if that's intended, a bug in the software running on the T2 chip, a bug in the NVMe implementation in Linux or if there is some logic necessary to properly initialize the T2 chip.