RomRaider

With the right tools/software, does anyone know if it's possible to write to RAM from a PC over K-Line while the engine is running without making any changes to the ROM?

I'm not aware of any method to do so with a stock ROM. Maybe with some of the SIDs I didn't look at much (A0, A3, 31...), but I doubt it.

Subaru k-line Hitachi\Subaru JECS ecu's allow data to be downloaded to any RAM area.
Subaru k-line Denso protects the stack area to prevent hacking.
Subaru SSM commands are B0, B8. No security is implemented.

You may check old Nissan's as well.

CAN ECMs the command is 3D, I don't remember the diagnostic session off the top of my head

If it means anything, UpRev implements their own custom SID for RAM writing for K-line ECU's. So you'll probably have to do the same.