Relocated path in a postinstall script - centos

Question

I'm working on an RPM package that deploys files to /opt and /etc.
In most of the cases it works perfectly, excepted that for a given environment, writing to /etc is not allowed ....
So I used Relocations in order to deploy the /etc files in some other location :
Relocations : /opt /etc
By specifying --relocate option I can deploy the /etc files into another location :
rpm -ivh --relocate /etc=/my/path/to/etc mypackage.rpm
Now the issue is that in the postinstall script, there are some hard coded references to /etc that don't get replaced when the package is deployed :
echo `hostname --fqdn` > /etc/myapp/host.conf
I hope that there is a way (macro, keyword, ... ) to use instead of hard coded paths in order to perform the substitutions during rpm execution.
If you have any information on this I'd really appreciate some help.
Thanks per advance
PS : Please note that this is NOT a duplicate of the previously asked (and answered) questions related to the root path re-locations as we're dealing with several relocation paths and the fact that we need to handle each of them separately during rpm scriptlets

Answer 1

Many thanks to Panu Matilainen from the RPM mailing list who answered the question. I'll re-produce his mail literally in order to share the knowledge :
I assume you mean (the above is how rpm -qi shows it though):
Prefixes: /opt /etc
The prefixes are passed to scriptlets via $RPM_INSTALL_PREFIX<n>
environment variables, <n> is the index of supported prefixes starting
from zero. So in the above,
/opt is $RPM_INSTALL_PREFIX0
/etc is $RPM_INSTALL_PREFIX1
So the scriptlet example becomes:
echo `hostname --fqdn` > $RPM_INSTALL_PREFIX1/myapp/host.conf
Works like a charm, thank you very much Panu !