Are we still talking about Reserve rolls or just whether there is a general principle that enemy models, can't benefit from special rules even when a special rule does not purport to limit itself to friendly models (as opposed to the general principle being that your opponent's models can't benefit from your special rules, if the special rule does not purport to limit itself to friendly models)?

If the former, I don't see how page 7 of the [url=http://www.games-workshop.com/MEDIA_CustomProductCatalog/m2590005a_40K_RULEBOOK_v1a.pdf]BRB FAQ[/url] is not dispositive. As already quoted:
Q: Do modifiers that apply to such things as Reserve rolls, apply to units from an allied detachment?
A: No.
That makes friendly vs. enemy, mine vs. yours moot as regards Reserve modifiers, doesn't it? Not even friendly allied models can take advantage of modifiers to "such things as Reserve rolls."