There are transit definitions which include writing specifically as being timely. One that I recall clearly is Saturn trine Mercury. It's a good time for one's best writing. Also it's good for contracts. The sextile is similar, and also the conjunction. Opposition and square are times to avoid such activities. When Saturn is in the third house it is a good period to write, especially where accuracy and detail are important.
Jupiter is similar when it transits in aspect to the natal Mercury. The conjunction, trine and sextile are good times, while the opposite and square are bad. Also as it transits the third house it is beneficial in stimulating good writing ability.
Then also, when Mercury transits some of the natal positions there can be good and bad effects, depending on the aspect.
Pluto transiting the third house or Mercury can greatly benefit writers also.
Then also when planets aspect houses beneficially there can be good and bad effects. So for example, not to bore you with a seemingly endless list, when Pluto is transiting the 7th house or 12th house, it beneficially aspects the third house.
And Uranus of course would bring a special genius thinking ability to writing when in the third house, or in aspect to Mercury.
Furthermore, looking at signs, the constellation that would apply to writing would be Gemini, also Virgo, ruled by Mercury.
etcetera