There are many ways to rig external halyards. And one of the fun things about owning a DS is to make the setup fit your preferences.
First, though, John's suggestions about adding flotation is excellent. Stuff in a pool noodle, or two from the top. I wouldn't limit myself to 2', but there's no need to stuff the entire mast, because the leverage is best at the tip. Pool noodles are better than expanding foam for this. With expanding foam you never know what you get, and it's a mess to deal with when it fails with age. Just make sure they are wedged so that, even if the mast top gets under water, they don't "float" up inside the mast towards the mast base.
Second, I'm personally not a fan of halyards lead on deck. Only time I've ever seen a main come down unplanned (under sail, no less) was on a boat that had halyards lead to the cuddy top. I've also never been in a situation where I had to drop a sail and couldn't do that by reaching the halyard cleats on the mast. If you already have the hardware set up that way or know and like that setup, remember, it's your preference.
Third, lines and suggested diameters. With modern rope design and materials 1/4" line is vastly oversized based on the required strength. With something like FSE Robline's Dinghy Control line, you could go down to 4mm and still have it be strong and not stretch. Similar with Amsteel from Samson Ropes. Both would reduce the "weight aloft", which helps counter the effect of the foam you added in step 1. Their difference is that Amsteel is very slippery (it has no cover) and therefore you'd need to splice a tail to it from some cheaper, heavier and grippier double braid. (That's a fun project, if you like it, and instructions are on L-36.com (look for halyard splice)). The tail would start about where the cleat is, when the sail is fully raised.
The Dinghy control comes with a cover and is grippy enough that you can use it as is. It will look weirdly undersized, like a shoelace, but I have it on another boat where it works well. (I also use it for outhaul and reef lines on my DS). For more suggestions on what lines to use where see:
Rope for various lines.
Fourth, I'm thinking of adding a 2:1 purchase to my jib halyard. Something to do with better control of the luff tension. My sail has an internal luff wire which could be tensioned to the point that it will actually off-load the fore-stay. Without purchase I can't get the halyard tensioned enough. Some people here suggested using a trucker's hitch in the halyard to get the extra purchase; for that you'd need a check block on the base of the mast. You tie the knot, lead the tail through the block, up to the hitch and back down to whatever blocks/cleats you have for the halyard.
Or you could cut the halyard above the mast bottom, and tie it to a floating block. Then lead a line up through the block and to the halyard cleat. This can be extended to a 3:1 purchase with an additional cheek block at the bottom of the mast. (See also:
3:1 Halyard Tensioner).