To clarify Sergio's find a little more - the actions to perform in the Win7 guest are a little different to XP. You don't search for new hardware as listed in the linked thread, as Win7 will never find it (or didn't for me). Once the serial port was set up in the host, the following steps gave me what looked to be a functioning serial port (can't test it as I'm lacking the hardware at the moment)
Go into Device Manager (not Add Device), easiest is to just type "Device Manager" in the search box in the start menu
In Device Manager, highlight "Ports (COM & LPT)"
In the Action menu, select "Add Legacy Hardware" and click next
Select "Install the hardware that I manually select from a list" and click next
Select "Ports" in the list and click next
Select "Standard Port Types/Communications Port" and click through to finish
You'll now have a new COM port, probably COM3. It will probably just work after a reboot, but if not, you can do the following two things (from the port's Properties page) to help play around:
Change from COM3 to COMx (if you want to) in the Port Settings tab under Advanced
Go into the Resources tab and select Set Configuration Manually. Untick Use Automatic Settings, select Basic Config 8 in the dropdown, then highlight IRQ or I/O range and click Change Setting. Give the values that are listed where you set the serial port in the virtualbox host software for that guest machine.
Like I said, haven't been able to actually test yet, but it appears to be a working port with no warnings, which is better than previously having no entry. Thanks to Sergio for finding the other thread for clues.