Call me stupid, but is it possible to use an IBM BladeCenter HS20 server as a standalone unit without the BladeCenter?
You can't run a blade server without the chassis, the connector is proprietary and replicating all the network and power hardware would be an encumbering task, as MikeyB suggests getting a BladeCenter S chassis will suit your needs perfectly.
Possibly, but you'll need a way to connect power to the server outside of the BladeCenter (power connections are normally provided by the BladeCenter), and you may run into problems with cooling when you try to do this (active cooling is generally provided by a series of blowers in the BladeCenter).
You'll also lose some of the redundancies that are provided by the chassis, and not built into the actual server. This may not be a problem for bench testing or configuration, but deploying a production server like this would defeat much of the purpose of specialized server hardware.
In short, I wouldn't recommend it. Get a 1U server that's not designed as a blade.