We have the same situation here. You need to have two different revision schemes, one for letters and one for numbers. It can be handled in one workflow. You just need to have workflow states and transitions that use the appropriate revision scheme. A data card variable could be one way of triggering which state to transition to. We use a transition for Major Revs (Letters) and Minor Revs (Letter and Number) or Prototype Rev (Numbers) at one of our sites without the need for a datacard variable triggering it. Instead the user chooses the appropriate state. See the image below:
Then here is our settings for sending documents through the Major revision for example:
Notice that the Revision _CIR_Alpha is being used. The drop down list is available to choose any revision scheme. I have it set to automatically pass through the state as well.
I hope this helps.
I have been working on this presumably in the time you were writing your response and I have been stumbling. I need that choice to be made automatically (no user choices). Your comment about having a data card variable to control it triggered me to realize why my workflow was stumbling. Although I had the custom property in my files, the data card was not updated with a place for the variable. I will try this!!! Thanks.