is it possible to reverse engineer a specific k2 version of some process at k2 workspace to produce the original k2 process?

I believe you can pull the complete project of the version of the process from the K2 Workspace Management Console by going to

{servername}:5555 -> Workflow Server -> Processes -> {Project} -> {Process} -> Versions

and then click on the Download link for the version you need.

pauld is correct.  When a K2 process is deployed, the process definition (KPRX) get's bundled up and pushed to the server as well and is available to be recovered from this location (a project file, KPRJ, is also included).  Note, this only includes the process definition (and potentially InfoPath form I believe, if you're using an InfoPath process).  If your process makes use of other things like SmartObjects or external assemblies, it will still reference these, but the actual VS artifacts (SODX, Class library project) will not be included in the package you download from Workspace.

One should not think of this Workspace functionality as a replacement to standard source control best practices.