I am part of a community garden in NYC. There are feral cats around and sometimes I see them in the garden. If there are feral cats already hanging around your community garden, you shouldn't need to attract more. There are no pests that afflict your garden that feral cats will control.
Source: I grew up on a small farm with a feral cat problem that took years of trapping cats for fixing and vaccination to get under control. I was wondering if it would be a good idea to set up a little cat village to keep them in the garden to scare away birds and other pests? And also that feral cats can be risky from a health perspective. You're just as likely to attract skunks and raccoons, which will not end well for the garden. Cons are that they dig stuff up, sit on stuff, and do cat things all over the place.
Even my house cats dig up stuff and poop and pee in beds making them not useable. I might try to attract native predators, like gardener snakes , sharp-shinned hawks , ladybird beetles, etc.
Feral colonies need to be managed by experts that can monitor their health, provide spaying/neutering, parasite control, and proper shelter and nutrition. The mere presence of cats will not keep birds away.