Mature PhD Project Application - How are these received?
I can't speak for any PhD program other than my own, but can I just say that I have a really hard time thinking of 27 as 'mature'?  FWIW, I didn't even get a BS until my mid-30s, and spent enough years in industry to become financially independent before pursuing a PhD.  Had no problems whatsoever* getting into the program.

(*But I should probably note that this is at the state university branch near where I live: getting into e.g. Stanford or CalTech might be a different story.)

While I'm not a lawyer by any stretch of the imagination, I think it likely that (in the US, anyway) it would actually be illegal to ask a candidate's age.  WRT to the particular project, I'd suggest getting to know people on the project.  Convincing the project director that you have something to contribute certainly can't hurt your chances of admission.  And if you're asked why the gap period, just state that you went into industry in order to make some money, rather than racking up more student loan debt.