Here is a blog someone posted about using gre(4) and ospfd(8) with IPSec. Since I don't use OSPF and I don't use GRE, I have not read more than the first paragraph.
I don't know if it will be applicable to your environment, nor do I know if it is either helpful or useless, correct or containing misinformation. It may be out-of-date, as it is from 2009. Use with caution.
http://journal.reallyenglish.com/200...h-openbsd.html