By Transpluto, do you mean any body that has influence that is outside the orbit of Pluto? How does a transplutonian object affect an analysis. Would you say an object, say Neptune, has more influence that a transplutonian object?
Honestly, I am not an expert on Transpluto - the astrologers who use it on a regular basis are the best ones to ask for more detail. There are two ways I use the word. Capitalized, "Transpluto" commonly refers to a hypothetical object beyond Pluto. In lower case, I am referring to a class of possible objects beyond Pluto.
I personally feel that generally speaking, the closer in to earth, the more relevant an object should be - not just because of proximity, but because closer objects move more quickly and are more likely to interact with the natal chart through transits. But are they more influential overall? I don't know that is necessarily the case - astrologers also study the galactic center and fixed stars, and they can be incredibly powerful indicators of personality in some charts. Like any astrological analysis, all these factors have to be considered on a case-by-case basis.
So what is the ephemeris for this Transpluto? If it was published in 1972, why do you still call it Transpluto? Surely telescopes are powerful enough to spot it now, especially since it should be large enough. I mean, if it wasn't large it wouldn't have any bearing on astrology, right?
Not necessarily. Chiron is a large asteroid, small planetoid, and it has a strong bearing on astrology. The Nodes are essentially mathematical points in space, not planets, and there are some astrologers who use them almost exclusively, giving highly detailed readings. There are plenty of objects too small for telescopes to spot - there is a solid history of astronomical objects being located after small wabbles are noticed in nearby objects.
As for the ephemeris, I believe it is no longer in print, but you can find them in some astrological computer programs, and may be able to find them in books on Transpluto on Amazon.com.
So, some people use charts with hypothetical positions? ... and by saying "likely has a primarily generational effect.", you mean it can also not have only a "generational effect", but other effects? That "transpluto" component may well have a combined mass far superior to all "inner Pluto" bodies. Do they not matter because they are far away? Even if there are billions of them? By your title "Bad Astronomer makes bad assumption ...", you assume Mr. Phil Plait assumed you would struggle to explain something about Pluto, that he would have a chuckle at your expense or both?
Many people use hypothetical positions, yes. Pluto itself is considered a primarily "generational" planet by most astrologers. It takes so long to complete its circuit of the zodiac that entire generations of people have it in the same sign. This does not mean it does not also have individual effects, but the strength of that effect will vary from chart to chart, depending on house placement and aspects to other planets. You could be right, a transpluto component could have a huge impact on some charts ... but I choose to wait until I know more before I consider it in my own analysis.
As for Mr. Plait, I do not have a direct quote (would be appreciative of any Coast subscriber who can provide one), but it was not an assumption, those were his comments more or less. He stated that he believed astrologers would struggle with this because it is possible Pluto might be declassified as a planet ... and that he was looking forward to having a good chuckle as a result. If its classification does not matter to him, why should it matter to me?