Need plugin to add footer to separate WP installs

I am building a portfolio of separate, stand-alone yoga-themed websites. Each one has a different complementary offering. For example, one site is a directory of studios and teachers. Another site is an online magazine. Another site is a Q&A site like Quora. Etc. Will be a dozen sites in total.

I will keep these as separate WP installs because I might want to sell an individual site at some point, so there will be less problem separating it. And, each site should stand alone - it offers just one thing - keeping things simple instead of trying to offer everything in one site. Also, if one site fails due to a plugin or theme issue that is unique to it, it won't affect the others.

I want to have a footer at the bottom of all the sites that lists all the sites in my portfolio. Want to be able to update and edit this footer in just one place and have it reflected on all sites.

I don't think the WPMUDEV Footer Content plugin will work for this situation.

Any suggestions?