Has anyone replaced this? Mine appears to have lost grip but I'm wary of drilling carbon masts and even more wary of riveting them again when the new cleat is in place.  I'm thinking about contacting local riggers and asking them but thought I'd check here first and see if anyone has done this and what the experience was.

Agreed, the thought makes me nervous too.
 
As a matter of interest, have you tried end-for-ending the halyard, to see if it's just wear on the rope? (run a mouse-line thru the top of the mast if you try this! 
 
Clive 

I replaced mine a couple of years ago. As far as I remember I just screwed the new one in with the same self tappers that were there for the original cleat. I haven't had any issues with the new cleat. My boat no. is 320. Maybe with newer boats the cleats are riveted. I did replace my outhaul cleat using rivets though. I used the special rivets for carbon fibre and it worked fine. No issues drilling with new holes in the boom either.
 
Emmett. 

 Replaced mine with using self tappers, moving the cleat up a little to avoid the previous holes. I used lots of inhibiting compound but after two years the stainless/aluminium junction built up a stalagtite of crud that pushed the bottom of the cleat about 5mm from the mast although I didn't realise it till inspecting this winter - the cleat was still doing its job.
 
Now I've put a nylon block inside the mast with threaded holes to line up with the screws.  Very tricky but it spreads the load nicely.
 
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 I've had to replace all the other camcleats on the kicker, cunningham, boom outhaul and tiller rudder-downhaul after all the original Selden trade mark cleats had corroded and lost their grip in only 3 years.
The Selden cleats appear to be just painted aluminium whereas you can get "Camcleat"  trademarked parts which are hard anodised and should last longer particularly in salt water.
Just had a look at my mast cleat and looks like that will need doing too.
Regards
 
Nick 
 
 

 interesting reading, I've end to ended the halyard and also since the last one buy a bit longer than needed so I can cut it down a few times and thus move the wear point.  In fact this is my third halyard in 4-5 years, as the outer and core parted company on the previous two, although last time that was down to trying to get the so-called two-part mast apart!  I am salt water sailing and perhaps I just have no luck with ropes??