I've tested practically every trem setting unit except for the Tremol-no(unless somebody wants to send me one *wink* *wink*), and actually have the ESP Arming Adjuster installed on ALL my guitars. If you don't want to route into your guitar, you won't like the Hipshot tremsetter either since you gotta drill a 1/2" deep hole on the back of your trem cavity to accomodate the bar.
That tremsetter from WD Parts is interesting, but basically just acts as a trem stopper and stops your trem from going any further back(which is bad if you pull up on the trem). So realistically your options if you want full use of the trem you'd have to go for an Ibanez Backstop or an ESP Arming Adjuster (or that Rockinger one)
I've actually successfully installed an Arming Adjuster on a guys guitar WITHOUT any routing. The only trick was to screw in the bolt in as far as it would go and also shorten the overall length using the adjustment on the back of the Arming Adjuster. Depending on how deep your route is your trem may not have a lot of back pull as the trem eventually hits the solid part of the arming adjuster.
Seriously, even though you can more or less install an arming adjuster without routing, I strongly recommend that you DO route it to properly fit one in. Especially since it's not a whole lot to route, and the benefits a backstop or arming adjuster gives is well worth the trade off. I mean I routed the cavities my J-Customs, my USA custom and my UV777bk, I must have been nuts to cut wood out of those guitars, but hey, it was worth it!
If you don't want to do it yourself you can always take your guitar to a tech to do it (or send it to me
(oh yeah, you can install the Arming Adjuster with either two springs in a V or straight(depending on your string gauge) or even three or four springs. You just don't use the middle one. Feel free to bug me if you got any questions about installing one of these buggers.