Methinks object->flatten transparency... is what you're after.
Or if that doesn't work out, (which in testing, seems not to all that well), ungroup and make a compound shape out of your stripes, then put your mask shape in front with no stroke or fill and use the pathfinder cut to cut all the stripes, allowing you to select (with the white arrow since it'll all be grouped) and delete the scraps.
Yeah, Flatten Transparency didn't do anything.
I thought the pathfinder route, but that's a lot of manual fiddling around deleting scraps. This seems like it would be a fairly needed function.
After you flattened the transparency did it have a clipping mask? Was it group how many times was it grouped?
What you are describing is just another example of the very common need to trim artwork (including filled, unfilled, open, and/or closed paths) without wrecking its already-applied Attributes, Appearances.
As has been discussed many many times here, Illustrator simply does not provide that basic functionality...neither by its horribly-designed cutting tools nor through its problematic and caveat-riddled Pathfinders. Expanding a Clipping Path doesn't serve as a workaround for this long-missing functionality; nor does flattening transparency. You can't do it without resorting to the workarounds you've already tried or others that will no doubt be suggested.
NONE of Illustrator's cutting tools (Knife, Scissor) can cut across artwork that includes multiple open, unfilled paths. Open unfilled paths are ignored by the Knife cut. The Scissor cannot cut across multiple paths at once.
Illustrator's recently-added, very poorly implemented Eraser tool comes nearest to the needed functionality. It can at least cut multiple unfilled open paths without their becoming converted to unstroked, closed fills. But it unpredictably alters the shapes of remaining portions and has no means by which to accurately control a cutting shape other than rectangular.
This is a prime example (there are others) of how Illustrator confuses and clutters the interface with multiple under-powered tools designed for similar functions, none of which measure up to a single tool properly designed for the same purposes in most other programs.
Pathfinders are riddled with ridiculous caveats. Those which come close to providing the needed functionality are incapable of ommitting the unwanted deadwood and/or wreck strokes by outlining them.
Illustrator needs a feature that allows any given path to be employed as either a selection marquee or a cutting path. And the cutting operation needs to act as expected regarding strokes as well as fills--as common in any other drawing program that does provide the long-missing functionality (most do; Illustator never has).
To meet the expectations of James, you may also consider this dirty destructive deed, with the masking selected:
1) In the Transparency palette dropdown select Multiply, or whatever else than Normal,
2) Object>Flatten Transparency with 100% Vector.