LDAP default value variable interpolation

I'm wondering if there is a way I can dynamically generate an LDAP default value by using variables.

I have a configuration that works something like this on my front end proxy:

pass_attrs = mail=user,
=nopassword=y,
=proxy=y,
=host=mail.%d

The above works perfectly well even though it seems a bit hack.

I want to fetch the host field from LDAP and default it to "mail.%d" if the attribute isn't populated for the user.

I have tried the following :

pass_attrs = mail=user,
=nopassword=y,
=proxy=y,
=host=%{ldap:mailHost:mail.%d}

however the interpolation doesn't occur on the default. Just wondering if there is a way to achieve this?

thanks