Not doing any work with BD lately but if you could expand on on your requirements for alphanumeric field we might be able to suggest some work around. Could you combine a auto number with a fixed selection of characters?
This is something I get asked for often. Basically you need to use a Maths/Logic Function to populate a variable. For example, RND(26) will generate a random number between 1 and 26 if you apply formatting to it.
You now need to convert the random value into a letter.
if Variable = 1, then Variable = A,
if Variable = 2, then Variable = B
if Variable = 3, then Variable = C
if Variable = 4, then Variable = D
And so on.....
Do this for all 26 letters of the alphabet and you will have a random single letter generator.
To have more random letters at the same time, just scale it all up. and merge the results of each single letter generator to whatever field you want to display the values.