Like escoville and Dragon, I (think I) have never seen or heard this word used before. However, now that I check, it is
in the dictionaries, and has been for decades (at least). And the (somewhat older) dictionary I have to hand does not
list the word as being rare.
OTOH, I have seen and (in relatively formal contexts) used the word "perspicuous." IMO, "transpicuous" is more readily and instinctively understandable to most people than is "perspicuous."
My dictionary says that transpicuous means "transparent; esp[ecially], easily understood."
My dictionary's definition of "perspicuous" is: "clear in statement or expression; easily understood; lucid."
The two words seem to be synonyms. Of the two, "transpicuous" seems more easily understandable to the first-time reader. IMO, add the word to Leo's dictionary, with a tag of "rare."