Finding the disadvantaged areas is the comparatively easy part. Many municipalities have maps of neighborhoods that have food insecurity, housing insecurity, an obvious lack of transportation, terrible graduation rates and test scores, high crime rates, and a lack of registered businesses that aren't primarily liquor stores or fast food.
Yes, it is hard to get businesses past the stigma of these neighborhoods. But what's the worst thing that happens? You offer the tax credit and nobody takes it? I think the bigger risk is if they take it, but the government doesn't step up to provide other services that are needed for these businesses to thrive.
No, I would not say having the government open offices is going to do anyone any good. But it could be beneficial to start a program in addition to this credit where the government can go into these neighborhoods and have classes where they teach people how to become local entrepreneurs (I think they do this already in a limited fashion in "nicer" areas)
Mainly my proposal applies to the urban poor, but rural areas still have some untapped opportunity. Take the rest of the world's rural poor as an example: small farmers organize into co-ops whose products are then sold on one or more markets (which depending on the market will be either horrible or great for them). I think in 20+ years when automated transportation starts to become a real thing, we'll be able to more cheaply scale out small manufacturing jobs to rural people. Ideally this could again use a co-op model, but I haven't seen many tech companies with that in mind.
Everything I have mentioned, btw, you could start with a private organization and then get government funding to cover a portion of your expenses. For example, most food banks and school lunch programs get most of their funding privately. They could also re-route some of the ridiculous defense spending back into food assistance programs, which we know is a benefit to local economies.