Directory + Custompress to generate alphanumeric string

Hi

I am using Directory and Custompress to build a directory of plants and I want to give each plant its own unique 11 character (string) identifier for the purpose of the directory.

Is it possible to use Custompress and/or Directory to create a unique 11 character alphanumeric string for each directory entry? It must be random and not incremental.

I hope this make sense