"Sometimes they are written down in a phoneticised approximation of a Scots accent, which makes them look more Scottish than they sound"

Marje there is the Tom Leonard style of poetry, like many Facebookers etc, who write phonetically - but I'd say Scottish songs rather than being written in phonetic spelling in order to make them look more Scottish - are more often than not simply written in a traditional spelling of Scots. There is not a written standard but there is a tradition of spelling. The various dialect of Scots don't just have their vocabulary and verb forms but their spelling system too! Actually it could even be argued that the written form of some of the ballads etc are possibly anglicised to make them seem closer to Standard English than they were! I'm sure Scott was accused of that.

Writers of Scots find it a problem sometimes and some words often tend to be written in a way that they maybe would be in England if pronounced that way rather than in a more traditional Scots spelling. I'm thinking 'hoose, moose, toon" etc

There have been various, not rules, but recent recommendations for writing in Scots published. First in the mid-20thC in the aftermath of the Scots Rennaissance and then more recently. People use all kinds of weird phonetic spelling on social media sometimes without realising there is a tradition of Scots spelling.