It seems to me that this might help lubricate the sole of the plane (like candle wax on the sole) for better smoothing
It would indeed. There's a long tradition of oiling the soles of planes, with a variety of oils (I think the main one historically in the West being raw linseed oil).
I don't see a good reason to use walnut or jojoba in preference for this purpose*, but within reason there shouldn't be a downside to doing so.
if you plan to finish a piece with walnut or jojoba oil anyway
As surprising as it seems you can actually lubricate plane soles with oil even when that oil won't be used in the finishing process. Same with wax which is the other common lubricant applied to planes to reduce friction (and a better choice IMO).
It is natural to assume that the oil or wax would contaminate the wood at least a little and cause a problem with the final finish but in practice this doesn't appear to be an issue.... although I'm sure there's someone who has had a problem with it sometime, but that's likely to have been user error.
might have some effects on the wood fibers cutting easier.
I've hand-planed reclaimed wood that was soaked in oil and it's only at that kind of level that I think you'd notice a difference in the wood.
But between a metal plane that has a completely clean sole and the same plane lubricated there can sometimes be a huge difference in the ease of planing because of the reduced friction. That effect alone may be enough that you won't feel you need to try oiling the wood itself but experiment away if you want, there are no rules about what you can and can't do in your own workshop!
*There are likely cheaper alternatives that'll work just as well. Certainly where I am walnut oil is expensive and jojoba extremely expensive.