The refhashlist gets automatically resigned in postinst/prerm. Unless the signing actually fails in Open Mode (accli return a non-zero status), installation should go fine. I assume the singing will go fine, although it obviously won't have any effect in Open Mode.
If the signing would fail however, the postinst/prerm will handle it gracefully.

P.S. you could inspect the postinst/prerm file yourself, the relevant commands can be found in INSTALL() and UNINSTALL() respectively. If you, or anyone else, sees something that would not work in Open Mode, let me know. I'll try to go into Open mode myself today, so I hope to know by the end of the day whether the (un)installation in its current form works in Open Mode or not.

Update: there are two flavours of Open Mode: one with aegis neutered ("patched" Open Mode), and one without. The current busybox-power installation works on both, but is less than ideal when running patched Open Mode. I need to create a function that can reliably determine whether aegis' source origin check is enabled or not and call aegisctl accordingly. Aegisctl itself won't do for this check, it reports the check is enabled in patched Open Mode.
The function is required since it's best not to call aegisctl to modify security options in patched Open Mode; aegisctl appears to fail when you try to run it a second time without a reboot in between. Stay tuned for updates.