Laptop keyboards are directly connected to embedded controller primarily because they are completely passive switch matrices (and EC includes - often directly in hardware - logic for scanning keyboard matrix), so there is nothing meaningful to exploit on the EC side.
Also connecting EC directly to some PCI bus does not make much sense from both system design and cost perspectives. Usual place to connect EC to is LPC, which is explicitly designed for such devices (things on motherboard like serial/parallel/game ports, TPM, FDC, keyboard controller/EC, BIOS flash and various ). Random review of datasheets found by google seems to indicate that chips that are only embedded controllers and do not contain additional ISA based peripherals (like ISA DMA controller itself) tend to not even implement the pin required for LPC DMA/bus master transactions (as it is not required for anything in normal operation).
Also connecting EC directly to some PCI bus does not make much sense from both system design and cost perspectives. Usual place to connect EC to is LPC, which is explicitly designed for such devices (things on motherboard like serial/parallel/game ports, TPM, FDC, keyboard controller/EC, BIOS flash and various ). Random review of datasheets found by google seems to indicate that chips that are only embedded controllers and do not contain additional ISA based peripherals (like ISA DMA controller itself) tend to not even implement the pin required for LPC DMA/bus master transactions (as it is not required for anything in normal operation).