If you managed to nearly collimate it, it would actually be hard to use unless you can fix both the TX and RX in position. (It isn't trivial to point uW dishes, for example.)
So I think you're on the right track to use as high-powered a TX LED as you can find. I wonder if you could co-phase them to get even more power -- that might be tricky for light. It is common in radio work though.
I too have been frustrated. We have one TiVo receiver, and it is in the living room. We sometimes wish to use it in another room, by switching it in and the room-specific receiver in. So I used a repeated too, but it is rather lame and spotty, as you say.
What I was going to do is add diplexers to the coax that I route the TiVo in, and shoot the repeated RF up that. In this way the RF has much lower loss across the cable than across the air. I don't know what the repeater frequency is, but I would just roll my own. I think
47MHz is that catch all band, but really, anything clean below channel 2 is fine.