> Can this easily be done from any Linux distribution?
Let's see...
> Network boot, using PXE (i386 or amd64) or diskless(8) (other platforms).
Red Hat and Fedora had this for ages. Also the installa media can be on a FTP, HTTP or NFS server.
> External USB CD-ROM or USB floppy, if your machine can boot from one.
This should work with most Linux distributions, too.
> USB Flash disk or hard disk
The same.
> Worst case, if none of the above is suitable, you can usually pull the disk out of the target system, use suitable adapters to install it in a "normal" computer, install OpenBSD, then replace the disk back in the target system.
Again this should be possible with most Linux distributions.
Now does OpenBSD have an automated installer?