I think you could do it engine in.
However how to mount it in an in between way I really dont know. I never really looked how the firewall is mounted now. You would need to avoid the harnass bar, but the roll cage is pretty far forward, the fuel pump opening is there, but you could make a part removable there.
I would think best bet is to attach to the top of the roll cage with a few spacers, then come down on whatever angle lets you avoid the harnass bar. And to use a harness bar that just uses the holes on the back of the roll bar, not the seat belt holes.
2007 Exige S with almost every track mod...