Sure, but there are some Sass placeholders set in both Studio and the LMS that you could extend to at least keep your z-index in line/scale. Our team has cleaned up a lot of specific z-indexes to use these placeholders, so extending these same placeholders would make your work more future and bullet-proof.

* [LMS placeholders for UI-depths/z-indexes](https://github.com/edx/edx-platform/blob/master/lms/static/sass/base/_mixins.scss#L92)
* [Studio placeholders for UI-depths/z-indexes](https://github.com/edx/edx-platform/blob/master/common/static/sass/_mixins.scss#L74)

Please try to pick the appropriate placeholder out and test if they can be extended to your xmodule work and that they get the job you want done.