Is Hagu's name a reference to The Hague?
While it is difficult to prove this with any certainty, we can try to analyse the facts around it to make an educated guess.
First, as far as I can find, the author of the manga has never said anything proving or disproving this relation, which unfortunately does rule out any definitive answer in either direction.
Do note that she has mentioned that the title of Honey and Clover comes from the album titles of the musicians Shikao Suga and Spitz, which can be seen as indication that she does make references.
However, the name Hagumi, and in turn the nickname Hagu, is common enough that inferring any significance from this alone is difficult.
A prime example from anime would be Hagumi Kitazawa from Bang Dream!, who is referred quite frequently as Hagu externally (and within the source material IIRC)
This is personal opinion, but if the author wanted to make a reference via character name, it seems much more likely that she would have used the name of a famous artist (perhaps a Japanese one, to avoid any finagling to form a plausible Japanese name) than The Hague, whose claim to international fame as an artistic city is frankly debatable.
The combination of a commonplace Japanese name and a tenuous artistic association to &quot;reference&quot; makes it, in my view, much more likely that this is a simple coincidence due to the sheer volume of names with some artistic significance.
Of course, without a clear statement from the author, all this is necessarily mere speculation.