Fade out the edge that wraps (as well as leaving the button hanging over the edge so that it's clearer that there is more unseen content. Clearer because not only is what is off screen not visible but a little bit of what is on screen is not visible as well.
Alternatively try and make out that it's like a conveyor belt or something that physically wraps round the corner or round the back.
And then you can do that on both sides if you have scrolled towards the middle and have content of of both ends.
And then remove it from the other end when you have scrolled to the other end.
Suitability of the conveyor belt metaphor
I would imagine the main interaction with this mechanism being via a left to right drag/swipe rather than via a tap on the left to right arrows. You may find that users might tend to tap the arrows before they discover the swipe gesture (depends on the user obviously) so it may be beneficial to make the arrows larger (more tap-able) and also to use icons which signify not only the direction of extra content but that it can be accessed via a swipe. (eg Luke Wroblewski's touch gesture guide)
There are some comments about breaking of design principles and scope for confusing the users. I would strongly suggest that in order to determine whether this mechanism is indeed suitable or confusing, then if being considered, the idea should definitely be tested with real users before release - as always.
Wrapping the content?
You or others may consider having it so that you always have this mechanism visible at both ends and actually allow the content to wrap back in from either end - but this would depend on how many items you have and whether it makes sense in your scenario.
It might be useful for example if you have some ordered content and it's significantly easier to wrap from the end over to the beginning again rather than have to scroll all the way back to the other end.
That sort of thing always infuriates me on things like personal video recorders where you can scroll though a list of recordings but to get back to the top you have to go all the way back up with about 50 clicks of a button, but I know it's not suitable for all situations!
But... Do consider if the scroll mechanism is really needed
All the above addresses the specific question as asked (ie wrt to making the buttons look more scrollable). I do wonder though, if the mechanism is even needed.
You've mentioned the extra keys are for keyboard keys not found on the iOS keyboard (for which reason I see absolutely no problem in using the same design as the rest of the keyboard, by the way).
It's worth checking out iA Writer shown below and which has had some excellent reviews - Eric Speikerman saying it was the best app on his iPad. Why - because it focuses on simplifying the task in hand - the writing.
iA Writer manages with just a handful of extra keys - including the very useful (larger) arrow keys. I'm sure iA could have added more keys, but retaining simplicity was clearly top priority in this beautifully designed app.
It's also worth noting from the perspective of visual appeal how iA Writer's top bar has the keys overlapping keys in the next row - rather than being aligned with them. This would be recommended in order to continue the flow of the visual design and blend more easily from the built in keyboard into the additional row. Even if you do use a scrolling mechanism. I'd suggest ensuring the overlapping design is retained. iA Writer has a pair of larger abutted keys which allow the design to reach the left and right extents without leaving unsightly gaps.