Hello thereisopenbsd, and welcome to daemonforums.org.
One possibility to access that partition is to edit the disklabel, and remove one of the other Linux partitions from it (preferably one you don't need to access), and replace it with the one you want to access. That means, change the offset and size parameters to define the partition you want.
While this is not hard to do, it can be quite scary the first times you try something like this. So, read the
disklabel(8) man page thoroughly, and proceed with caution (double- or triple-check everything). If you make a mess, simply exit disklabel without saving changes, regroup, and start again.
I'm not enough of an expert to suggest anything else if this method isn't suitable for you.