This thread has been of particuliar interest to me because I revamped my lanyard system about four years ago to be able to do this same very thing, moving from the dee ring over to the front of the saddle to incorporate a mini SRT system to gain another point of attachment while climbing.There are some advantages to this system even though a climber may not need to employ it on every climb, as has been said it is nice when the situation calls for it.
Personally, I don't see the practicality of using a prussik in this manner when a sling would be a better choice. Why would anyone want to lose lanyard length choking it off in this manner, a sling can give you some additional length and avoid the issue of carabiner side loading. It would just as easy to rig a sling and hook into it as it it would be to take the time to tie a prussik onto your lanyard.
My experience with this set-up has been that you use it in the outer portion of the crown of a tree where choking and side loading a carabiner can damage it because your tying into smaller diameter limbs and branches. With your full or partial body weight bearing down on a choked carabiner or snap your just asking for trouble and damaged gear.
Larry