I feel like most musicians can probably write a melody/lead line (whatever you want to call it) that is better than something they can improvise on the spot. I think improv can definitely be a powerful writing tool, but why not improv over x amount of bars until you work out some cool parts, cut out the noodling, and just choose the best parts.. rinse and repeat until you have a cohesive 'solo' that is fully worked out and arranged?
that could work unless the splicing makes it feel fake