I don't recommand a hub between. Normaly OF creats the right path to a connected USB-Device, but this depends on OF-Version (the device MUSST be inserted before booting OF). Older versions does'nt do that and you have to set path manually, which is a bit tricky.
If there are more USB-Devices connected to that hub, you may loose bandwith, which may slow down the system.

If you heve created your bootscript, yo need to change some env-variables in OF:

To see the actual settings type >printenv

following entries have to be set:

setenv boot-device hd:,\menue
setenv boot-file menue
setenv boot-command boot

Menue is the name of your bootscript, which has to be located on first (hfs) partition of boot-device (typically the same place where boot.img is located).
It makes sense to do this on first partition of internal HD (hd:,\).
Every time you boot now, this script is excuted first.

Here a example of a bootscript (which has to be adepted for your requierements):

\ FORTH - Script generated by BootCreator 1.2 (25.01.2007) by Marcin Kurek (Morgoth/D-CAPS)
\ Mac mini adaptation - Marek 'March' Szyprowski
\
\ terminal control stuff
\
: .printf fb8-write drop ;
: .crlf " "(0d 0a)" .printf ;
: .clear " "(0C)" .printf ;
: .cr " "(0d)" .printf ;
\
\ boot menu stuff
\
: my-max-boot-num 5 ;
: my-boot-default 1 ;
: my-boot-delay d# 100 ; \ unit = 100 ms
: my-print-menu ( -- )
.clear
.crlf
" Wellcome to PowerbookG4/PPC 5.8, registrated to Dipl. Ing. Harald Schnabl, A. Merzgasse 15-17, A2380 Perchtoldsdorf, Austria" .printf .crlf
.crlf
" ======= Boot Menu =======" .printf .crlf
" " .printf .crlf
" 1: MorphOS" .printf .crlf
" 2: MacOS (from USB)" .printf .crlf
" 3: CD-ROM (insert CD)" .printf .crlf
" 4: MorphOS BootMenu" .printf .crlf
" 5: OpenFirmware" .printf .crlf
.crlf
" Please press 1-5 or wait for auto boot" .printf .crlf
;
: my-boot-case ( num -- )
.crlf
case
1 of " boot hd:,\boot.img ramdebug BD=Boot" eval endof
2 of " boot ud:,\\:tbxi" eval endof
3 of " boot mac-io/ata-4@1f000/@0:3,\\:tbxi" eval endof
4 of " boot hd:,\boot.img ramdebug BM BD=Boot" eval endof
5 of abort endof

endcase
;
: my-input-num ( wait-period max-boot-num default-num -- boot-num )
1 \ loop-inc = 1
3 pick 0 do
" ." .printf
\ 0d emit
\ ." press 1-"
( wait-period max-boot-num default-num loop-inc )
\ 2 pick ascii 0 + emit
\ dup 1 = if
\ ." within "
\ 3 pick i - d# 10 / .d
\ ." seconds"
\ then
\ ." (default: "
\ over ascii 0 + emit
\ ." ) :
d# 100 ms
key? if
key
( wait-period max-boot-num default-num loop-inc key )
dup 0d = if \ return pressed
drop leave
then
ascii 0 -
( wait-period max-boot-num default-num loop-inc num )
dup 1 5 pick
( wait-period max-boot-num default-num loop-inc num num 1 max-boot-num )
between if
rot drop swap leave
then
( wait-period max-boot-num default-num loop-inc num )
2drop 0 \ loop-inc = 0
then
dup +loop
drop
( wait-period max-boot-num boot-num )
nip nip
;
" screen" output
dev screen
" "(0000000000aa00aa0000aaaaaa0000aa00aaaa5500aaaaaa)" drop 0 7 set-colors
" "(5555555555ff55ff5555ffffff5555ff55ffffff55ffffff)" drop 8 15 set-colors
erase-screen
device-end
f to foreground-color
0 to background-color
my-print-menu
my-boot-delay my-max-boot-num my-boot-default my-input-num
my-boot-case