Broadly speaking most of だけ's usages fall under two general categories: a negative sense and a positive one. I think because the negative sense is taught earlier to us Japanese learners, most people are more familiar with that sense. But the other meaning is also very common and unfortunately easily confusable with the negative sense.
The negative sense functions to limit a degree, scope, or reference, and is explained as:
- 範囲を限定することを表す。「～ばかり」、「～のみ」。(Wiktionary Japanese)
in a negative, limiting sense: only, just, limit (Wiktionary English)
But here the other sense is invoked in your text:
in a positive, non-limiting sense: amount, as much as
As explained in that Wiktionary entry:
異国で執筆が出来るだけ: Doing/achieving as much as 異国で執筆が出来る (being able to write in a foreign country)