[HTTPS-Everywhere] Transformations for regex capturing parentheses desirable?

[J.B. Nicholson-Owens]

I would like to capture a substring and transform it in some way in the 
substitution:

   http://no.exist/foo/BAR123  ->  https://no.exist/foo/bar123

Notice how "bar" becomes lowercase.

So I would like to use the following ruleset:

<ruleset name="no.exist">
   <target host="no.exist" />

   <rule from="^http://no\.exist/foo/(.*)$"
         to="https://no.exist/foo/$1" />
</ruleset>

but somehow specify that $1 should be altered in a specific way: 
lowercase it.

Is this desirable in more places than my particular ruleset?

Is there a desirable way to do something like this (I'm thinking another 
attribute on the rule element indicating which variable to transform and 
how to transform it)?

Thanks for your feedback.