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You could try slicing, with a dremel or similar instrument, a 'notch' into the bolt that would allow a flat edge to fit in. Soak the bolt/area with a penetrating lubricant (PB Blaster or some such) or ammonia, let it sit and do its work for a few minutes (some folks recommend 24 hrs), and then attempt to 'tighten it through' the eyelet. You could also try ...

Bosses in aluminum frames are typically rivet nuts, aka "pem nuts", which have a head that should be too large to slip back into the frame. For your frame, it sounds like the Park Tool directions for securing bosses on carbon frames should work equally well. The top mount on the seat-tube is also the easiest to repair using this method, so you're in luck.

No matter what you do: if you have loose metal parts hanging around inside your tubes, then your first priority is to get them out. The Park Tools link in the other answer has some good ideas. My first thought would be to get a metal nut or threaded sleeve and epoxy it in place. But the simplest option is to not fix it at all, and just use a clamp to ...

Take the bike to your local automotive repair/machine shop. Bolts into a blind hole with the head sheared off is a common automotive problem. They will have tools to take care of this problem. 5mm is on the small side for automotive bolts so you may have to contact more than one shop to find one with a screw extractor (aka easy out) for 1/8" or 3/16" ...

A long time ago I used some clamp on cable housing guides on a frame & I didn't have any issues with them moving. I did have a slightly larger cable housing with teflon sleeves inside, so that the clamps could be screwed down fairly tightly and still allow for easy movement of the cable inside. You might want to see if you can get perhaps another ...