I think your idea works for reducing any shock loads that may be imposed on the jackstay fixing points, however we take the view that the idea of jackstays and harnesses is to prevent anybody going overboard in the first place so we keep ours tight and as close to the centerline as possible with shortened lanyards. If conditions dictate we will re route the jackstays inside of the shrouds as they are the adjustable webbing type.
Any crew member going out of the cockpit in rough conditions (me) keeps their body weight very low (I have been spotted on my hands and knees!) which reduces the risk of going overboard in the first place and once at the mast we rig a lanyard around the mast to clip onto that which means you can’t even get swept out beyond the inners.
The shortened lanyard I use when it’s rough means at worst case if swept off my feet whilst going forw’d should keep me on the boat, albeit clinging to the toe rail at chest height.
I may end up a bit bruised but I would rather that than risk having to get back on board as a potential MOB.