There's a whole market of suppliers that's been going strong for decades. Especially for safety-critical embedded with untrusted, Linux/POSIX apps in separate partitions. QNX is one of best examples far as commercial adoption:
EDIT to add the QNX Desktop Demo that came on a single floppy. Throwing it in since you were mentioning resource-constrained systems further down. A floppy is 1.44MB with base QNX running in ROM's of embedded systems. Can scale such architectures it up or down however you wish. :)