Parsing User=$TOTO as "the User is the value of the environment variable TOTO as given by Environment or EnvironmentFile" might be a reasonable feature request, but it is not currently an implemented feature.
I think anything that simplify transitioning from an init system to another new one is valuable as long as it does not breaks the original new design intention.
There has been a good and valuable effort trying to split original upstream packages provided init system scripts by debian developers into /etc/default/X and /etc/init.d/X file and storing most commonly changed sysv init options in the default file part (including start or not). Wasting this work due to systemd transition would be a bad idea IMHO.
Being able to reuse the sysv default config file or automate its back and forth conversion (like passwd and shadow or group and gshadow with grpck and pwck) would be helping both init system transition for debian devs and administrators that have been developing their own sysv scripts.
Looking at ways to replace start-stop-daemon various parameters by corresponding unit file sequences/file layout would probably help (-g and -c are just examples for their User and Group systemd counterpart)
--eric