Considering the shape of the torso/head, the lower tubes are dangerously close to the jenner's own body. Just looking in mechlab you will notice that they would barely clear the antenna coming from the head, and would zip directly over the top of the head. Might work if the mech were firing from a stationary position....,
However, the Jenner is designed for speed, jumping, and maneuverability, and would most likely shoot itself with missiles launched from such a location.
Battlemech 101! Don't shoot yourself in the head with your own missiles!
Edit: This is regarding the lateral fire of SRMs. I realize that LRMs make sense there and were probably the intention (looking at the Oxide default configuration).
If stacking on top of the top launcher looks ridiculous (probably would) how about two side lanchers (think the side pod on atlas 3rd missile slot) up by the head?
Edited by LoneUnknown, 04 December 2013 - 11:36 PM.