Can one just supply JUNOS image, FreeBDS image (if necessary) and simply place the box in the network diagram all within one PC, link to Cisco virtual boxes and be happy with that?
It is as it works now. For doing this you need to create and run multiple virtual machines - qemu images. Each qemu image (VM) contains FreeBSD with JunOS installed. That is why we need to use Qemu if we want to run multiple Juniper routers on one PC - virtualization is needed.
Another possibility is using VMware for running multiple VMware images with FreeBSD and JunOS installed but VMware is not integrated in GNS3. If you want to use VMware, you need to connect Junipers to Cisco emulated router via cloud (loopaback or tap interfaces).
As I said Qemu is integrated in GNS3 - it means you can control (create/start/stop/delete) your Qemu Junos images directly form GNS3 desktop - no need for doing this from cli. Everything what you need is to create your base-image (http://blog.gns3.net/2009/10/olive-juniper/
and set Qemu and JunOS parameters (path to base image, type of interfaces, RAM, Qemu acceleration). You do it only once. After that you can drag and drop Juniper routers to GNS3 desktop, start routers and qemu (qemu-img), controlled by qemuwrapper create, start Juniper for you.
If you don't like virtualization and you have many old computers you can install FreeBSD and JunOS on these computers and connected them via Ethernet cards. It will works too.
All my tutorials can be found here:http://brezular.com