How can I invest in bonds (either directly or indirectly) that meet specific, project-based criteria?

Most corporate bonds do not go to specific projects, they are used as general funds by the corporation.

Municipal bonds, however, can be "Earmarked" for specific projects or purposes, often for school districts, or for infrastructure projects like highways. It can be hard to find them, though. There are some bond screeners that you can filter by name and other attributes (or ask your favorite AI chatbot), but you'll have to look at the prospectus of the bond to see what the money is actually used for.

One drawback of individual bonds is that their face value is often $1,000, and since munis have very low default risk, they trade close to $1,000 each as well. I do not know any brokers that offer "partial" bonds like they do stocks.

There are also bond funds that will hold hundreds or thousands of municipal bonds, but they will be very widely distributed and may not fit your narrow criteria.

As a side note, understand that buying a bond on the secondary market does NOT impact the issuer of the bond in the slightest. They already got their money from the initial offering (just like when companies issue stock). So unless you are in on the initial offering, and no one else would buy any more of the bonds except you, your investment, while maybe easing your conscience, is not directly impacting the project or organization it was intended to fund.

If you want to make a direct impact, then donating to charities or causes that you believe in will be more impactful.