Also the line gets flattened pretty easy by the pressure from the cleat - what I do is I push a smaller piece of line inside of the halyard so that it keeps its 'round' shape - this will keep it from popping out too - the halyards get flatting and then they tend to come out - really bad when that happens and someone sails right over the top of you - it happens to everyone... just gotta get that halyard set up with a jacket or something to give it some structure - be careful putting a cover on it - since it needs to slip through the bock at the top - so can't be anything to hang up on - really needs to be a small piece of line spliced inside if you ask me.
I've never thought about putting something in the dyneema. It would be quite a bit cheaper. The only downside until it wears in it is really slippery and tough to grip. I spliced a few feet of dyneema into the retrieval end of my halyard to prevent some wear on the spinnaker, and that last pull was tough for a while, you had to do a wrap on your hand to get a good grip.
I use a 6mm Robline line, (I think its "Dinghy Control") then pull the core mostly out and splice the cover back into the core.. From just after the cleat to the head of the spinnaker its the 3mm dyneema core. Then a short section where its the cover and the core in the cleat, then for most of the retrieval side of the line its just the cover. Saves some money too, as you pulling the core out and making the one line longer sorta. Its not a huge savings, but it helps when that stuff is $1.25+ a foot. For whatever reason just the cover is really easy to grip
My halyard cleat is a Harken H468, mounted on a Ronstan Swivel. Stay away from the Ronstan cleats in this application. The Harken has a metal strip in the cleat base that prevents wear. The Ronstan does not, and eventually the halyard will wear into the cleat base and get trapped above the jaws and you can't drop the chute and the last leeward rounding, when in first place, and beating out some rockstars....... Don't ask me how I know.