Is this inspired by any particular micro-kernel design?

Looking at syscalls.h, it looks like it abstracts the platform details, for example.

Is SYSTEM for amd64 source-compatible with the riscv version?

There's ideas in here from various other kernels - some of the capability stuff is inspired by (but simpler than) seL4, and the message passing IPC and how I'll avoid priority inversion is based on ideas from QNX. Generally as a learning process I've tried to keep as open a mind as possible, without trying to reinvent all the wheels at once...

SYSTEM has a tiny arch-specific assembly trampoline which just sets up the initial stack pointer for new processes that it creates and makes the jump, but other than that it's source-compatible across architectures.

The platform detail extraction isn't yet complete, such that devices management on non-ACPI platforms isn't finished, but the idea is the abstraction will be enough that drivers for (at least) MMIO devices will be trivially portable.