Questions about isolated code lacking context are closed as 'missing review context.' However, I have a question that, if another piece of code were part of, would count as review context. Suppose I rewrote that specific piece of code and want a review but don't want to upload the whole code again because I want the new question to focus exclusively on a specific part. Can a link to the old question count as 'review context?'
What you're effectively asking is whether it's ok to post a follow-up question which uses the original questions as context, so you won't have to repeat yourself.
While questions should stand on their own for all kind of reasons, in practice links towards other post on the network get somewhat more leeway. Links towards questions on the same site even more so. After all, they are easier to view, safer to view and less likely to rot compared to the average outbound link.
Will everyone gladly follow links? No. There is some risk involved with regards to reduced likelihood of getting answers. Do not rely on links too heavily. However, considering follow-ups have been posted often enough before (we even have a FAQ dedicated to it) and some projects even split up in 2-4 parts to focus on different parts of their project in different questions (with varying degrees of success, it doesn't always work out well), there's hardly a rule against it. The FAQ provides some guidance on what a question should contain to avoid being a total disaster. While a single follow-up is usually ok, there is such a think as too many follow-ups.
Putting as much as possible behind a link is not always the best choice, not at all, but there is no rule against keeping some of it in an older question. The more related they are, the better. Stray too far and you'll get in a grey area attracting votes to close while reducing the chance on an answer.