on a wider scope it seems to have been a suboptimal decision to reuse this directory for different unrelated subsystems like fanoush's bootmenu and uBoot bootmenu.
Terribly confusing to everybody tying to understand what's the nature of uBoot and bootmenu, and obviously even confusing uBoot tools.

I'd at least have defined a dedicated /etc/uboot-bootmenu.d/ and symlinked it to /etc/bootmenu.d/ in case I really want to do this "one dir for all" approach. Now it's probably not exactly simple to fix all this.
Usual approach to fix: tools first check for existence of /etc/uboot-bootmenu.d and fall back to /etc/bootmenu.d/ if the former doesn't exist resp isn't readable (incl symlink to a readable real dir).
(I boggled and frowned on this part when I first read it, now it turns out my intuitive reaction was obviously justified)
/j