It looks like it might be off to Registry Editor to set environment variables for Windows services (I'm unsure if there is a UI for this hidden somewhere 
):
Looks like you can go to HKLM\SYSTEM\CurrentControlSet\Services\<service name> and define a Multi-String registry key called Environment and put individual <name>=<value> definitions on each line. The key wasn't there for me after a fresh installation. Upon submitting, it produced a warning about stripping empty strings (even though I tried to not have a blank newline in the value data) but seemed to work just fine.
With Windows you could probably even distribute this via Group Policy if your machines are domain-joined.