The phrase 書くだけでいいですか, would be "Would just writing it be okay?" We can addも to make it slightly more polite: 書くだけでもいいですか. The politeness nuance comes from softening the suggestion by implying you're not "married: to it.
The pattern or TE/DEMO means "even" and can be used with nouns and some particles like だけ。
鉛筆でいいですか - Would a pencil be okay?
鉛筆でもいいですか - Would (even) a pencil be okay
鉛筆だけでいいですか - Would just a pencil be okay?
鉛筆だけでもいいですか - Would (even) just a pencil be okay?
