Can you help to identify these frames, bikes or parts?

the top tube dent is creased

That's not great, but the dent is less than half the diameter of the tube so it wouldn't concern me. But it will reduce what you can ask for it.

The rear dropouts are unbranded. The fork ends seem to have 'GIPI' written on them

As suggested above, that'll be Gipiemme. They were a "second tier" Italian manufacturer, ie if Campagnolo are Premier League, Gippy are First Division: good, not great.

Seat post is roughly 27mm, I don't have a caliper to get an accurate measurement. The seat post it comes with looks pretty mean..

Roughly 27mm suggests it's a roughly decent tubeset, which you'd expect with Gippy ends. The post might look mean, but the clamping arrangement is shit. Which isn't to say you can't pass it off for enough to buy a decent one.

Would there be any way to fit a rear brake and a rear hanger?

Yes. You could pay through your nose for a clamp-on brake, which attaches to the seatstays and there are a number of hangers that locate on track ends.

I know that sounds silly...

Yes it does. If you want a bike with gears and a rear brake, buy a frame that takes gears and a rear brake, it will be much easier and almost certainly cheaper in the long run.