Is there any chance you can try this with a soft keyboard on the Mac instead? I've always had problems with remote Ubuntu installations over IPMI and lower-end ILO solutions.I usually have to resort to an on-screen keyboard on the ILO/IPMI side or on the local OS side.
(sorry, I can only give these instructions in English...)
Open the Language & Text pane of System Preferences, click on Input Sources, and enable both the Input Menu and the Keyboard & Character Viewer.
You can enable the keyboard viewer from the top menu.

This will present an on-screen keyboard like the Key Caps accessory from the old Mac days.

When you get to the keyboard detection spot during the Ubuntu install, try inputting the requisite characters this way.
If this results in the same behavior, you can work with the keyboard settings in the HP Remote Access Card. Toggle the settings under the General menu:
General: You can control the keyboard pass-through mode and select
pass all keystrokes to pass your management station's keystrokes to
the remote system. Some keystrokes are intercepted by the management
station OS and will not be passed on.
Or if you are really desperate, you can try programming a keyboard macro in the IPMI that outputs the character you need. Of course, check the Internationalization settings of the Remote Access Card (if any exist).