Seems the Informix agent is after all available for 5.0. I did a search for VCS Informix Guide - one of the hits was the LBN TechNote for 5.1:
http://seer.entsupport.symantec.com/docs/282004.htm

It contains a link to 5.0 Agent Guides:
To access the current VCS 5.0 MP3 documentation for the database agents, use the following URL:

http://www.symantec.com/business/support/overview.jsp?pid=15066

5.0 Guide:
ftp://exftpp.symantec.com/pub/support/products/Clu...

According to this manual, the Informix agent is contained in the ACC Library package.
p. 16:
To install or update the ACC Library package, locate the library and related documentation on the agentpack disc.
p. 17:
To install the ACC library
1 Log in as superuser.
2 Navigate to the pkgs directory (the pkgs directory on the CD).
AIX: cd_mount/aix/application/acc_library/vcs/version_library/pkgs
HP-UX: cd_mount/hpux/generic/application/acc_library/vcs/version_library/pkgs
Linux: cd_mount/linux/generic/application/acc_library/vcs/version_library/rpms
Solaris: cd_mount/solaris/dist_arch/application/acc_library/vcs/version_library/pkgs
where dist_arch is sparc or sol_x64.

3 Install the package. Enter Yes if asked to confirm overwriting of files in the existing package.
AIX   # installp -ac -d VRTSacclib.rte.bff VRTSacclib.rte
HP-UX   # swinstall -s ‘pwd‘ VRTSacclib
Linux   # rpm -i \
VRTSacclib-VersionNumber-GA_GENERIC.noarch.rpm
Solaris   # pkgadd -d . VRTSacclib

4 For HP-UX, install the HP-UX patch PHCO_29042 if it is not already installed.