Interesting problem – when you say that autodiscovery won’t work, that sort of implies that the firewall(s) are extremely tight.
Have you tried any programs like SolarWinds.net management and discovery apps? You can download a 30 day trial package – chief limitation being that you can’t save the results to a file – but then it still may give you enough structure and information to form the basis for an outline.
Since you say that you’re a tech writer, I don’t know just how technical you are (or are not), but even so, I’d ask the engineering group (or who ever owns/runs the network) for the output from a number of standard queries.
1) Get a copy of the routing tables – this will show you what networks are in use – whether they are directly attached or not.
2) If you use cisco routers, ask for the results of a “show cdp” command (cdp=cisco discovery protocol) from at least a key router at each site. This will also aid in describing the network.
3) Ask for a list of known sites of the organization. This will help you to possible build a hierarchical structure to your maps.
4) Be prepared to discover things that the engineering staff will tell you aren’t true, or didn’t know about.
None of these things will help you to build your base map automatically, but (speaking as a veteran of many mapping exercises)they will help you to get a flavor of what you’re facing.
p.s. Shameless self-promotion: If you need help, and are willing (your employer, I assume) to pay for it….