I was under the impression that we had a TSC-based timer and could switch
via sysctl or a loader variable, but maybe I was confused, probably it's
FreeBSD 5+.

Out of curiosity, does setting hw.i8254.walltimer to 1 or 2 change
the situation?

Do you need the patch only during the boot?  If so, I'd rather try to fix
the cputimer, or add one that works for VirtualBox.

Cheers.