Normally, when using 9 slices, there’s some option for repeating the interior tiles. In UMG it seems they are stretched only, with no option for tiling/repeating. Is this the case? Is there any way to tile/repeat them?
Yeah, that’s about right. You’d have to use 2 elements to have a scalable border with tiled interior.
A border set to Border (or Box). And an image on top set to Image mode which supports tiling.
Shame, Box mode should have an option to tile the middle part, it does not:
You can always do it all in material. 
I might be mistaken, but even the border option stretches the edges, doesn’t it?
The picture in the documentation is incorrect/misleading. The description is fine, though:
-
the Box draws a 3x3 box where the sides and the middle Stretch (orange arrows) based on the Margins (dotted blue lines)
-
the Border draws a 3x3 border where the sides Tile (orange arrows) based on the Margins (dotted blue lines).
To be perfectly honest, I never managed to get the border mode behave as I’d expect. It always left me wanting. Perhaps there is a specific type of image it works with correctly.
Generally, from my experience you want to have the base image as small as possible - the min size the border will ever be and only allow the border to scale it up. Although this might sounds trivial for someone accustomed to 9-slice already.