> For example, can I use the C backend to compile zig to C, and then use the system compiler as I would normally do with a meson cross file or CMake toolchain file?

It's possible, but:

1. The C backend isn't 100% there yet. You won't be able to use all features and might run into bugs.

2. The generated code won't be very readable, it's arguably not too different from just using Zig-compiled object files directly in terms of "opaqueness" and legibility.

If neither of these are a big problem for you (both points are likely to improve with time), then yes, you could do that.