Not to drag this out further than we've already come, but here is a video on splitting wood. There are a few things that he does, that I learned not to do.
If you get green wood, either throw it in a rain barrel right away, or slather the end of the log with wood glue to slow down the drying process... I prefer the rain barrel.
Splitting wood with a froe is much easier than using hatchets. (Lee Valley Tools, or they can be made from a truck leaf spring) At 21:50 he uses a Japanese tool to split off the sapwood... the froe is very similar in size and shape, but the handle allows you to apply pressure (to one side) to have been able to split that sapwood off cleanly... but it takes practice.
Apply physics when splitting any wood... if you keep the same or similar mass on each side of the split you're making, the crack will run true (unless you have poor wood). (I see he did explain this about 13:20)
At the 12:00 minute mark he is trying to work around a knot... for your purposes this is a total waste of time and takes a lot of skill, and the result will always be unsatisfying... cut that stuff out.
To me, walnut IS an exotic wood... IMHO, you're stepping over gold bars, to go pick up horse turds... (But you are right, if you want the darker heartwood, you'll have to pick on a bigger tree.)
Wood will always split increasingly easier as the temperature drops, plus you don't sweat as much.
If you're splitting longer pieces (like 4 footers), wedges are handy; often, even wooden wedges suffice. Contact with metal will discolour some woods (particularly woods that have high tannin content, like oak. Given that you intend to draw the billets through the draw plate, you might want to keep this in mind.
Unlike this man, you probably only want to work with pieces that are 18" - 2'... it's a LOT easier the shorter the log is, and given that your dowels are not likely to be more than 3" in length a 2 foot log would work.
By the end, his video pretty much covers the topic... splitting saplings with a knife wouldn't be a lot different.
By the end, his video made me smile, because he's making axe handles. By the time that I made one or two of them, I was pretty good at swinging an axe or a hatchet. And once I realized how much effort it takes to make one by hand, I was much less likely to break one in the first place.