Using velocity for long term planning of a project is a wonderful idea. With minimal effort we get fairly good estimates. At least this would be the case in ideal situations, meaning new team without maintenance or obsolete component replacement obligations etc. If you are a believer in true teams as I am, you do realize that all the urgent work and even absence due personal issues affect team dynamics. Just calculating velocity in terms of team member availability isn't realistic with true teams. The velocity, or team achievement, is not linear function of sum of available team member hours. If one person is for example pulled away from team activities for taking care of urgent manufacturing issue, this will affect the team dynamics. This has deeper impact than just missing those hours. To be honest, in some teams the impact can be positive as well. In that case I suppose they were non functioning work group, which now due the changed dynamics has a chance to become a team.
This said, no, we have not solved the issue of maintenance and other urgent work disrupting normal work. We are struggling with bouncing velocity. This is for several reasons, but it also woke me to think about true teams again. Anyway, I'd like to pay more attention to this and give true teams a chance to grow.