Really?
Actually this topic is very close to my heart, my dialect is the biggest part of my identity! And I am very proud that said dialect is always in the top 3 of polls about which one the people like to hear the most !
Whilst there is indeed no set standard, like with High German...there have been books about Swiss German Dialects and their grammar and spelling since the beginning of the 20th century.
At least for Bärndüütsch, there have been quite a few books been issued about some rules to follow to make a text more homogenic and easier to read. And quite some of the rules, I follow for my published scribblings too.
One of the esteemed Bernese dialect writers, Werner Marti has already in 1972 published a book called "Bärndütschi Schribwys", followed by an even bigger and better,
"Berndeutsch-Grammatik für die heutige Mundart zwischen Thun und Jura . Francke, Bern 1985".
Also the writer Rudolf von Tavel, whose Swiss German books were published 1901 to 1933, has set a certain Standard many follow to this day.
His speciality was also to fine tune between the plain city dialcet and how the ̈patricians" (nobility) spoke.
Of course these are only rules set by a few esteemed writers and only for the Bärndüütsch from the city of Berne......
In my Canton alone, with the Emmental, Frutigtal, Simmental and Saanenland to name but a few, those books are more or less unusable, simply because of the richness of their own dialects and their own words, in the aforementioned valleys, which have often not much in common with the dialect in the city.
And this is the main problem to issue a standard for written swiss german.
It is simply impossible to boil it down to a standard for the german speaking Valaisian as well as for the Basilean, Appenzeller or Bündner.
So, even if there are no set rules, written Swissgerman is not an impossibility and 'not written' per se.