Yes, from an MBR partitioning perspective, you are SOL. You did not account for your final But not from an OpenBSD perspective, as long as you are aware that you are about to "go beyond" the MBR partitioning schema.
The -best- option is to repartition, properly. (BTW, your extended partition extends 63 sectors into cylinder 54945. That's a waste.)
Realize that OpenBSD uses MBRs for booting, for finding the OpenBSD partition, and for finding any foreign partitions. But OpenBSD partitions (a-p) are not stored in the MBR, they are stored separately, in the disklabel.
You can create disklabel partitions that reside anywhere on the drive, the disklabel(8) program uses the OpenBSD partition by default. You can change that default. Look at the man page for disklabel, and the "b" command.
If you have the time, it is best to repartition, and clean up your mess.
