Given that the /similar method can be sorted by relevance there must be a numeric rank for each question. Would it be possible to have access to the value? Off by default, but can be enabled by using a filter?
There actually isn't much sensible that we could return as a "relevance rank", what we essentially get out of Lucene is just the order of posts to return.
Internally, relevance ordering doesn't make sense to compare across separate invocations of the same query much less across different queries or even sites.
Our search infrastructure is also subject to essentially constant tweaks, so we couldn't even guarantee the semblance of stability in such "relevance ranks".
So, I'm status-declined-ing this as implementing it would be more or less the same as returning
rand() for each post in the result.