Thank you all for your suggestions.
In the end I managed to use Universal USB Installer, choosing "Try Unlisted Linux ISO (New Syslinux)" and choose the install ISO image to transfer to the USB pen. This USB pen would boot up the Red Hat installer fine, but the installer can not find the files for installation because it expects a CD/DVD with unpacked files, or a HDD with the ISO image (and the USB pen is recognized as a HDD).
The solution for me was to take out the USB pen, put it in my windows machine, format it with FAT32, copy the ISO image file directly to the USB pen, put it back in the machine running the linux install and let find the drive under "harddisk".
I believe it would also be possible to copy the ISO image to the USB pen togeather with the install files if the USB pen has enough space, or use 2 USB pens (one with install files, and one with ISO image).
Hope it will help someone else in the future :)