The problem I'm having in my mind is this - What is the practical limit of droop on a "stockish" truck? It seems to me that you have 2 choices.
1) run a longer lift spring with a longer shock that allows for down travel, but have to load the crap out of your truck to avoid the stink-bug look due to inability to lift the front.
2) Use shorter springs than in 1), but risk having your springs unseat during articulation.
Am I underthinking this? The inability to play with the front end really limits what your practical droop in the rear can be.
100% correct. That's why the lift height options are so limited. This is also what limits shock options (room for the shock, room for the shock travel, etc).
So with the front being limited to 2.75" lift, you'd want to keep the rear close to that same amount or ya...stinkypoo bug.
Shock-wise to run Billy's or regular OME you simply bolt on and go. They work with the available spring combos however you do not gain travel.
If you want to increase travel (up, down, whatever...travel is travel) you have few options.
*The rear is the easier way IF you are willing to lift your 100 to 2.75 front and 3.00 rear. In fact, look how many have their 100's at this height. A lot these days. So instead of bolting in a standard length shock you can easily bolt in a 80-series L-shock and gain 3-inches travel at the wheel. This will be far better on technical trails as the pics show.
If somebody can forego the aded travel because they need another type of performance increase then another shock might be better.
NOTE: At a recent show Slee mentioned his new shocks will have longer travel (droop) than regular OME though a bit less than the L. Overall it might make for a great shock for many users. For me...I will wait for the new L-series 80-series sport shock. I want every inch for the trails and don't need a reservoir.