This is super cool! It would be nice to have a simple C or Pascal frontend that generates Bril. Playing with optimization and code generation is a lot of fun, but it's hard to really dig into it without being able to test things on real-world, non-trivial code. You can use Clang and work in LLVM, of course, but that's an order of magnitude too complicated for an intro-level compilers course, IMHO.
I also like the decision to not have the IR always in SSA form. I think it helps you understand SSA form better to have to construct it, and you're going to have to eliminate SSA form anyway for any of the classic techniques for generating machine code.