So here are the diagrams we have covered:
- class diagram
- sequence diagram
- state machine diagram
- activity diagram
Still to go are:
- component diagram
- use case diagram
Based on the above it appears that BPEL could be described in a non-BPEL fashion using the activity diagram. An activity diagram could simply be described as a flow chart. It supports the definition of activities, looping, conditions etc. Sounds kinda like BPEL doesn't it.
Another diagram that appears to have some use is the component diagram. I am not 100% sure but the component diagram typically shows components in a system and how they interact with one another. Looking at Martin Fowler's publication UML Distilled (Third Edition), pg 141:
"Use component diagrams when you are dividing your system into components and want to show their inter-relationships through interfaces..."
Additional details in the publication and diagram examples appear to provide a way to visually describe a SOA where Web Services are the components.
I haven't researched this with other organizations but will trying to apply these prinicipals/approaches for some upcoming projects to help in the detailed design documentation phase.