I'm curious if anyone's ever played it like this, or if I'm trying to be too much of a rule lawyer... Wording of the spell is 'This spell gives the player a chance of re-rolling dice ... These re-rolls may be used for ... armour and ward saves' Link to the GW site with handy little card printout: http://www.games-workshop.com/MEDIA_CustomProductCatalog/m2050140_Lore_of_Heavens.pdf Now, the way its worded to me... it's not necessarily specifying that it's the player's own rolls that need to be rerolled, so you could actually have a ward or armor save that your opponent is making be re-rolled... (I'm also curious, is it by convention that people roll their own armor or ward saves, or is it explicitly outlined that that's the way its done).
It might be worth checking the FAQ's to see if it is covered there, but I think the fact that it says 'the player may reroll dice' twice in the two paragraphs strongly suggests you cannot make opponents reroll successful rolls.
p. 29 "Taking armor saves" "Roll a D6 for each wound suffered by your troops..." You definitely roll saves for your own troops, and trying to roll someone else's with a spell meant to reroll your own makes you a Limburger Lawyer. I wouldn't try it. In general GW doesn't hide "easter eggs" inside spells and abilities... sometimes due to insufficiently specific grammar you can find a technicality that a rule could mean something else. But it's generally better to play by the clear intention of the rules, as best you and your opponenet can agree upon. However, it is useful to question how rules are written right after new books are released. The people who write the FAQs seem to sometimes be different from the ones writing the books. Sometimes the FAQ will make a "clarification" that in no way reflects something that could be inferred from how things were originally written. Terradons gained the ability to drop rocks during a charge in this way.
on the subject of second sign... this is probably a stupid question, but when using a second sign re-roll, is that an individual armour/ward/to hit/whatever, or a whole batch of them? description of re-rolls on page 'XVI' (as in right at the start of the book) sort of hints at rolling all of them again...