I am delighted to report that the Maplins "USB Adaptor AF/min USB 5pin M", part A80AH, works with the N800 running OS2008 and automatically switches it into USB host mode - I plugged in a USB keyboard and it worked straight away!
The guy in the shop was nice enough to let me plug it in to prove it would fit (I should have tried a USB keyboard out in the shop, but I didn't think to do so!).

I get quite a lot of babbling in "dmesg | tail -200" about USB, but it seems to work just fine...
[62269.343750] musb_stage0_irq 663: BABBLE devctl: 3d
[62269.406250] usb 1-1: new low speed USB device using musb_hdrc and address 8
[62269.539062] usb 1-1: skipped 1 descriptor after interface
[62269.539062] usb 1-1: default language 0x0409
[62269.546875] usb 1-1: new device strings: Mfr=1, Product=2, SerialNumber=0
[62269.546875] usb 1-1: Product: HP Basic USB Keyboard
[62269.546875] usb 1-1: Manufacturer: CHICONY
[62269.546875] usb 1-1: device v03f0 p0024 is not supported
[62269.546875] usb 1-1: uevent
[62269.546875] usb 1-1: usb_probe_device
[62269.546875] usb 1-1: configuration #1 chosen from 1 choice
[62269.546875] usb 1-1: adding 1-1:1.0 (config #1, interface 0)
[62269.546875] usb 1-1:1.0: uevent
[62269.546875] usbhid 1-1:1.0: usb_probe_interface
[62269.546875] usbhid 1-1:1.0: usb_probe_interface - got id
[62269.546875] drivers/usb/input/hid-core.c: HID probe called for ifnum 0
[62269.562500] input: CHICONY HP Basic USB Keyboard as /class/input/input5
[62269.562500] input: USB HID v1.10 Keyboard [CHICONY HP Basic USB Keyboard] on usb-musb_hdrc-1
[62269.562500] drivers/usb/core/inode.c: creating file '008'
[62269.562500] hub 1-0:1.0: 100mA power budget left
[62269.562500] hub 1-0:1.0: state 7 ports 1 chg 0000 evt 0002
[62269.562500] hub 1-0:1.0: port 1 enable change, status 00000303
[62271.421875] tusb_source_power 629: VBUS a_wait_vrise, devctl 3d otg 172 conf c0010000 prcm 00a80500
[62276.039062] tusb_source_power 629: VBUS a_wait_vrise, devctl 3d otg 172 conf c0010000 prcm 00a80500
[62297.085937] drivers/usb/input/hid-core.c: can't resubmit intr, musb_hdrc-1/input0, status -19
[62297.101562] usbhid 1-1:1.0: retrying intr urb
[62297.132812] usbhid 1-1:1.0: retrying intr urb