You are correct, SCM knows nothing of Speccy specific hardware. As far as I know there have been no ports of SCM that would help you.
In theory, you can use any input and output devices for SCM's console I/O, including the speccy's hardware (screen and keyboard). However, to achieve this there must be SCM compatible drivers for those devices. If the speccy's normal firmware could co-exist with SCM it would just be a simple code shim to transition from SCM device driver format to speccy format. I'm guessing you will not be able to have them co-exist, so you would need to create SCM drivers with all the functionality required to control the speccy hardware directly. If you have to do this from scratch it would be a big job. If you have source code to support this functionality it should be quite manageable.
If you could add a 6850 or Z80 SIO based serial port from the RC2014 range then you could get this working by just assembling SCM with an appropriate RAM address for SCM's workspace, or possibly without any modifications. Once you have SCM running through a serial port you can use SCM to help you write and test drivers for the Speccy screen and keyboard.
I would be interested to know how you get on.