how do mimic the behavior of services symfony Tags?

You can extends phalcon di class and do whatever you want and implement this feature, no problem here.

I think your recommendation is the only solution.

Tags are very useful services.

If you want them in phalcon they obviously not to be implemented by someone. Do NFR on github. But copy paste link from symfony docs is not enough, because not everyone is using symfony and get the idea.

Also you can just implement this stuff for incubator in php. Then someone can do this for zephir.