download iso extract files or mount iso & copt files
install refind ( which creates its own partition ! ) disk
and create 2 partitions (as is advised in USB_INSTALLATION.txt ) with eg gparted
one
fat16 containing /boot and /EFI (copy from iso)
one
ext4 containing /porteus (copy from iso)
reboot!
this works for me

... i think if an "efi" "driver"? for ext* exist??! for and was included with syslinux
then all files/dir's copyed from iso could exist on one ext partition !

perhaps its just as simple to write a similar grub2.cfg ( which afaik can load/work from /EFI on ext partition *on my hw =P *ymmv )