as far as I'm concerned there is nothing hacky about using footer markup for its intended purpose, that is placing stuff at the bottom of the page. Rather any other method would probably involve lots of volatile hacks. In my book a hack would be abusing some functionality in some unintended way to get unsupported functionality. Using footer markup does not seem like such a thing. Of course you could change Lilypond's whole page layout system to effectively have two footer markups, but pretty much the same can be achieved with setting the footer markup to something that matches what you want.
Indeed, Lilypond's text processing capabilities are rather limited.
I'm still convinced it is a hack. Commonly, the "hack" term is used for indicating a work-around with some emphasis.
In the case we are talking about, David's suggestion would be a simple work-around (---> improper use of a label to bypass the problem). But the copyright field has an issue (which should be reported to Gitlab, I think: feedback are welcome, so I can report it): it is coupled to the footer but it is not necessarily a footer. In fact, a footer is not simply an element that is placed on the bottom of a page. It also has to be recurrent in order to be a footer. And a copyright field is not required to be recurrent.
Therefore, using a footer (and not the copyright field) as a non-recurrent string, so to align an element at a desired position is more than a work-around. I normally use two labels in my code (which is a common choice too): FIXME (for issues and hacks) and WORKAROUND for work-arounds. If I used the hack, I would write a FIXME label in the code: but it is likely it won't be fixed in the future, because the LP tools for cover pages have some expected limits that I think should be overcome with tools focused on text typography. Note too that the "footer" solution has unwanted side effects: for example, I normally print page numbers on the header of the page; but another good choice would be to print them centered on the footer as page X/totPages. I should have the flexibility to switch on the fly from one choice to another, which is expected in a header + body + footer template, therefore this template should not be polluted by mixing body with footer. As said before, an improper use of the copyright field would be an acceptable compromise, but it has the issue I wrote above.
Therefore, I conclude that using a tool for text typography (for cover pages) in addition to LilyPond is an exahustive choice for managing a complete score.