That half worked, the environment variable has been set (I can log on to the server and echo the value) but if I try to use the environment variable from a script called by buildmaster then the no value is returned. Why can't I use this value from a script called by buildmaster?

Pulling in a variable value from an external source would require a custom action using the BuildMaster SDK. These are relatively easy to write, but we can of course help with that through professional services (http://inedo.com/company/professional-services). In the action's overridden Execute() method, all you'd need to do is: this.Context.Variables["VERSION_NUMBER"] = GetVariableValue(); where GetVariableValue() is a method you create to pull the value from whatever source you want (could be from Artifactory, or something like File.ReadAllText())

Variables have been expanded quite a bit in BuildMaster 3.6... so this might be accomplished through a Promotion, Environment,etc. variable

This is not possible with that setup. If the DLLs are deployed separately at times, they should really be their own deployables (or even applications in some cases), that way you can specify them to be deployed at the release level. However, for your case, you may be able to get away with creating multiple variables (e.g. FILEMASK1, FILEMASK2, FILEMASKN, etc.) I'm not actually sure if that will work or not, and still wouldn't solve the problem if you have more than N files.