No. It was considered for both Stratolaunch as well as the SS1/SS2 programs, but it turns out that if you're trying to go as high as you can, there's no energy left over for a pitch up. And if you stay lower so that you've got excess energy for the pitch up, the angle isn't much and you don't get back what you lost from launching lower. Horizontal launch at the highest possible altitude turned out to be optimal. And in fact, you don't want to go TOO high, because then the wings on the rocket need to be very large in order to effect the gamma turn.I thought there was already a pitch up lob plan for the jet only version of Stratolaunch, wasn’t there?
Since the plane needs a dedicated launch facility to support it, and there are a VERY small number of airports out of which it can operate, I'd be extremely surprised if it's repurposed for anything other than rocket launches, and since it won't be doing that...I think the best chance of success for the SL is versatility of mission capability. Orbital launch missions I think will be few and far apart, but the ability to carry a variety of loads would allow it to be used in other ways such as fire bomber, modular cargo pod carrier, or even a replacement for the Edwards test drop mothership. You could even use it for unusual or bizarre applications like citywide leaflet drops, record setting formation skydive drops, rocket section carriage, large fish repopulation drops in lakes, large scale paratrooper invasion force deployment, large sample statistical test to check the likelihood that cats will land on their feet, and shade at airshows.