Can you describe your process in a little more detail? It's not entirely clear to me what you are after.
Plugins are for executing code. There are some nice code documenting tools like BaseElements but your approval and lockdown process needs some more info.
github is the standard for tracking SVN software development.
If you're looking for something where your clients are involved, you might want to take a look at basecamp:
I agree with Wim, you need to provide more details, not sure that a filemaker solution is actually what you're looking for, but it's definitely not a plugin.
The Advanced version of FileMaker Pro provides the Database Design
Report (DDR) tool which spits out a complete description of your
solution in HTML and XML. It's up to you to interpret the XML.
BaseElements by Goya does that. It includes the ability to compare DDRs,
so it may be the tool you want.