BIG EDIT:
Disregard below.. The USB partitions are now showing up in early startup

I think it's very tricky when you press both mouse buttons to get into the early startup...you have to do it at the exact right time...but anyway I am booting from USB now

Faster than internal IDE :-)
in this short thread chris explains what has to be done, just ignore my posts.. http://www.amiga.org/forums/showthread.php?t=49677&highlight=deneb+boot
Thanks for the tip. I read the thread. Ok so I have confirmed my USB mouse is working in the boot menu.

Still can't see my connected USB flash disk in early startup, though, so I can't boot from it. I have this setup in the Deneb flashrom:
-all of poseidon 4.3 (from quick setup menu)
-TLSFMem
-romupdate.idtag
-exec.library
-morelibspace (for AFA_OS)
-FileSystem.resource
-denebdmausb.device
-input.device
-bootmenu
-NoClick
-NewAlert
-FastFileSystem (version 45.15)
-icon.library
-workbench.library
-shell
-Shell-Seg
-Ram-Handler
-console.device
-scsi.device
I have my USB stick formatted with FFS, which is also in the RDB. I have one partition marked bootable with priority -3 and my workbench copoied to it. (on purpose it is set at -3).
I notice when I am powering on my usb hub (which is working reliably, btw), it does not power on until right after the "green" status screen of the Deneb goes away. Then if I quickly press both mouse buttons (on usb mouse) I can get into the early startup menu. I can use the USB mouse, works great! But I don't see my USB Flash disk that is connected and powered on via the USB hub. I only see the volumes of my IDE hard disk.
Anything else I need to do?
All the ROMUpdates in the Denen Flashrom are from BoingBag 2 or newer.
In Luciferin I have "Bootloader" option set to Re-install on every boot", "left mouse button" and "Status as Colour".
(Yes Colour is spelt correctly the Canadian way! LOL)
