Raspberry Pi port, piCore-15.0 is available!
You cannot mount a device in both the host and a virtual machine.. You will have to use a network file system such as NFS if you wish to share a"Folder" among multiple machines.Please spend some time in the wiki. I think you need to read http://wiki.tinycorelinux.net/wiki:virtualbox
It is possible to run a whole virtualized guest system entirely on top of QEMU's 9p pass-through filesystem (Documentation/9psetup) such that all guest system's files are directly visible inside a subdirectory on the host system and therefore directly accessible by both sides.