I was waiting to see if anyone responded to this question before throwing out some thoughts. I'm sure that are folks out there who have more knowledge about this subject that I do.
First, as to cutting - my sense is that pen blanks are a peripheral product that result from the milling of '1 by' lumber. That is, timber is milled into standard '1 by' dimensions (which are 3/4" thick), and then if there is usable lumber left over that doesn't fall into standard dimension lumber sizes, it can be further milled down to form saleable 3/4x3/4 pen blanks rather than just being thrown away.
But if you are starting with a tree (trunk or branch) and your only desired end-product is only pen blanks, then you are likely think about 6" lengths, and cutting patterns that maximize the number of blanks that can be produced allowing for wastage such as the pith, bark, and saw kerfs. So you would probably cut a 3/4" slice directly through the pith, and then cut additional 3/4" slices in parallel. Then, each of those slices would be cut into 3/4"x3/4" billets or blanks.. This would be a standard 'plainsawn' cutting pattern which would be fine since the blanks would eventually be turned so that the nuances of quarter-sawn or rift-sawn would be meaningless.
As to the value - there are several factors that influence the value of pen blanks, including the appearance of the wood, its scarcity, and any emotional added value that might be derived from the history of a specific piece of wood. In my opinion, poplar is not an especially interesting wood - it's bland without very interesting grain patterns, the color (greenish tan) is not very pretty, and it's fairly soft. That's why its the hardwood that is most often chosen for painted applications rather than applications that emphasize the natural grain and appearance of the wood. Its relatively common and it grows like weeds (and in my yard, we actually consider it to be something of a weed and try to get rid of it). It is among the softest of the hardwoods, and it sometimes can be pretty stringy. So the question is whether the fact that the trees from which this wood was harvested were originally owned by Thomas Jefferson would add much value. My suspicion is that history would not be a big factor, and pen blanks from this wood would not be especially valuable. Now if you could document that the wood came from a chair that Jefferson actually sat on, it might be more valuable.
I did poke around to see whether any commercial pen blank suppliers offered much poplar, and most of the poplar I found was stabilized and dyed - in other words, post-treated to remediate the natural deficiencies of being soft and bland.
