There's no such feature, and I think the reason is that it would be very rarely useful.
If your package only needs this dependency for its
preinst script, then
Install-Depends would allow users to remove the dependency after installing your package. This would only save
disk space, which is not likely to be an issue (Debian packaging rules are devised for desktop/server-type machines, not appliances with a few megabytes of flash). It would also have the surprising consequence that a user who wanted to reinstall your package would need to install the dependency (today reinstalling a package whose dependencies are satisfied doesn't require installing dependencies).
Furthermore, most install-time dependencies tend to occur in
postinst scripts, because they tend to be the most complicated. And a dependency for a
postinst script is not just for install time, it's for life: the
postinst script may be called to reconfigure the package (look at the Debian policy manual to see when the various package scripts are called).