Agreed -- the reliance on Linux is definitely something that we've found to be a mixed blessing.
We actually looked fairly seriously into other candidate OSes as starting points for building DIOS. We considered L4::Fiasco, xv6 and Barrelfish, and investigated Barrelfish in depth. (L4 and xv6, at least at the time, were available for 32-bit only, which clearly wasn't going to cut it for a warehouse-scale OS.) Ultimately, we went with Linux over Barrelfish due to the more comprehensive driver support, better documentation and the fact that it works on our test machines.
Barrelfish had great trouble booting on one of our machines with a lot of physical memory; after patching it, we managed to boot it, but did not have PCIe devices available (required for networking). To their credit, the Barrelfish team were very supportive, but progress was slow and working with Linux allowed us to get started right away.
However, using Linux has also bought us into a bunch of annoying non-scalable implementation choices. For example, DIOS relies on fast memory mapping, but Linux serializes all access to the mm_struct for a process using a single semaphore.
That said, DIOS is (deliberately) written in such a way that it should be possible to port it to other host kernels. Specifically, the kernel code consists of three parts: 1. a small patch to route the new system calls to handlers; 2. a BSD-licensed core module that contains the DIOS logic, but which is implemented in standard ANSI C and does not rely on any Linux-specific kernel features; 3. a GPL-licensed "DIOS abstraction layer" (DAL), which offers access to the Linux kernel facilities for process and memory management, VFS calls, etc.
While our current prototype is for Linux only, we intend to revisit the Barrelfish port and will also look into porting DIOS to a BSD OS in the future. Barrelfish especially should be interesting -- it's a very good fit for DIOS's abstractions.