here is a document with a statement regarding Oracle9i AS and MC/SG which matches exactly your problem:
 
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PROBLEM:
An Oracle 9i Application Server is installed on a two-cpu cluster in a 
non-standard Service Guard (SG) configuration, causing some of the Service Guard
commands to lose functionality.
 
BACKGROUND:
There are 2 issues involved:

(1) Oracle requires that the Application Server be installed on the shared
drives. The stated reason is that when the App Server package moves from one
system to another, it would have problems accessing some DB reports if it
was installed locally on each system.

(2) Oracle also configures the installation of the application to use the
hostname of the system on which it is installed, so that when the app server
starts, it verifies the hostname with the license key before allowing it to run.

...
 
In this configuration, the hostname is set to the "floating" hostname on one of
the 2 systems, then the application is installed on that system on the shared
drives, then configured to change the hostname of the box which runs it to
the "floating" hostname.  That makes App Server work on either box in pkg,
however it also breaks ServiceGuard because the hostname of that box is
NOT in agreement with what is configured in ServiceGuard. So commands
like cmviewcl or other commands will fail.   This arrangement may also cause
other applications to fail as well.



CONFIGURATION
HP-UX 11.11Superdome 64-way
Service Guard
Oracle 9i Application Server
 
RESOLUTION
In this case, the only way to allow the Oracle App Server to work while
NOT breaking ServiceGuard (not to mention other products) is to install
the Oracle App Server locally on each system while the box has it's own
hostname that will be configured in ServiceGuard.  Then, have Oracle provide a
list of which files that get updated in the installation directory at the time
the application runs (these files need to be moved to the other box in order
for it to run on the alternate box if the pkg fails).  Then write a script
(this can be run by a cron job) that copies those files on a regular basis from
the installation directory on the primary box where the pkg is running to some
directory on the shared disks (perhaps every 2 or 3 minutes).

The application script can also be modified to copy those files from the shared
location to their 'local' location in the installation directory, so that the
application is able to run smoothly on that box when SG fails over to it.

Oracle has suggested that this arrangement will work, however they should be
contacted for any other considerations to be aware of.

DISCLAIMER

Note that the suggestions provided herein are offered as a workaround and not a
complete solution that would first have to be tested.
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==>> the problem is in the design of Oracle9i AS !!
it is not supported to change the "hostname" online, if the application provider (in this case Oracle)
does so, it is their responsibility to take care of all the drawbacks (like breaking Service Guard);