Note: This uses the vcs_info from zshell version 4.3.11. If you are running 4.3.10, you can copy the vcs_info software from the 4.3.11 distribution of zsh and install that in place of what was shipped with 4.3.10.
The VCS_INFO*fossil files should be copied to prefix/share/zsh/version/functions/VCS_Info/Backends. prefix is probably either /usr/local or /usr, depending on your system. version can be found by running zsh --version.
Then, as root in the prefix/share/zsh/version/functions directory, run:
setopt extendedglob nomark_dirs zcompile -U -M VCS_Info.zwc VCS_Info/*~*.zwc(^/)
Usage is pretty much the same as it is for any other backend for vcs_info - see the zsh docs for information.
The misc field (%m) is currently set to the repository. Suggestions for alternatives are welcome (data from the fossil status command preferred).
Final note: if you're reading my blog entry for this code, it's changed enough that the vcs_info config at the end no longer works.
Bugs, complaints, suggestions, etc.
Best to open a ticket at the chiselapp repo for the software.