Sorry to interrupt. Why do you think a boot scan only scans one volume? The idea of a boot scan is to try to scan vulnerable areas/processes/files that are part of the boot process (of your OS, yes) before your Windows GUI is available for you.
Other volumes (partitions) and other OSs installed in the same hardware (multiboot) are not part of the specific boot process before Avast. This means that whatever is there in those other partitions and OSs can be scanned by Avast without the risk that some malware is hidding itself before Avast can take action (scan those other areas). That is, if your Windows OS can also have access to those other volumes (partitions).
I’m not saying that Avast would be able to scan and identify “everything” of “any” other OSs in your multiboot configuration, but since other OSs are not being part of the boot process (in that specific boot cycle), then a malware in those other areas can’t be hidding itself before Avast.
Of course, I might be wrong too :).