On veterans, several YC alumni, after working on the healthcare.gov rescue team, went on to help start the US Digital Service (https://www.usds.gov/) and Nava (navahq.com), which are working on the VA backlog. Both organizations are hiring.
Within the broader tech community, companies like Nuna are working on Medicaid and the VA backlog. On policing, DJ Patil--former Greylock Data Scientist and one of the founders of Color, FFS--launched an initiative around police data and criminal justice during his time at the White House: https://techcrunch.com/2016/09/13/dj-patil-criminal-justice/
And, of course, Code for America in SOMA does lots of work with local governments and you can apply to be a Fellow: https://www.codeforamerica.org/
There's a lot more to do, and not all of these efforts are as well-known as they should be. But if you're looking to help, people in tech are working on these problems.