Different runes have different agro. I'm going to make a guess that agro problem enchanter is having is because of self runes not because of runes being cast on tank. Just have enchanter use 101 self rune instead of the twin self rune spells and odds are agro issue will vanish.
Just some quick testing to add all runes are effected by silent casting so subtlety should work as well. I would avout casting self runes Enticer's Unity and Polyrefractive Rune on raids unless you are certain that you wont get agro. Poly in particular gives just enormous agro while Unity is enough to draw agro from adds if cast at wrong time. Other runes don't seem that bad at all in regards to agro,
Max SC is 100% aggro reduction so you really can't compare that to a % reduction given by SCS (esp. if SCS isn't working on single target rune aggro b/c of how the spell is treated). I'm not saying you are wrong but I still wouldn't agree that normal aggro rules apply to some runes. Enticer's Rune OR Poly are not a problem by themselves; however, Unity has a built in hate mod on top of whatever aggro you get from the rune. I do think it's questionable advice to suggest you should severely limit casting Unity on raids though... like others (and you) have said, if you're smart about it (ie. knowing your class) you cast it when appropriate. For me, that's more often than not. In Arx, Unity aggro has only been an issue in two situations [for me]- Arx 1 if adds are running around crazy (in which case I'm happy to take aggro), and Arx4 in the small transition from phase 1 to phase 2 (fixed by fading). This sounds 110% like a strat problem, not an Enchanter-can't-control-rune-aggro problem (no offense). Are you guys trying to balance the big golems down?
I did some more testing plus some spell parser research and things are a bit more complicated then I originally thought. Poly and Enticer;s runes don't have a hate mod so there potential agro is equal to the amount of damage the rune absorbs. Unity has a hate mod of 5000 so that is the max agro that spell will generate. So unlike my original recommendation I would infact use Unity over Enticers. Important note that I wasn't aware of is that casting runes are not 100% certain be noticed by mobs on your agro list. It seems casting runes is noticed only 25% - 50% of time. Runes also don't seem to increase your hate once you reach 100% agro level.
Actually runes give double their absorb value as hate. And basing this on some testing i did with my enchanter box (on accident actually), Unity rune gives substantially more then 5000 hate. I don't know about the noticed 25 - 50% of the time part, I havent tested this so just guessing, but i would guesstimate just like healing, based on the mobs intelligence score there is a weighted roll for the rune to give 0 hate.
5000 is the hate override on the base spell, you garner hate for each of the components on top of that 5k.
I didn't bother to max Silent Casting, or SCS for that matter, because I never have aggro issues, even on raids. I haven't touched current raids, so maybe I'd want it there? Anyway, if you want me to do some quick, easy testing, I can help. I didn't test or parse anything, so I always presumed the different reactions to my runes were based on the different actions of the tank, etc., but I don't know.
Based on my completely unparsed opinion, I see more aggro in Arx raids than previous raids with Unity. However, that could simply be due to the need to cast it more frequently and the higher HP values of the runes compared to previous versions. With max SCS it's still rarely an issue unless it's wipe time and then I'm usually in line shortly after the tanks... but that's really nothing new.
If rune agro works like heal agro as speculated, then level of agro can be adjusted by zone simply by giving mobs higher int stat.
