First - thank you for this excellent plug-in! I have wanted to simplify the add-post screen for my site members (a lot of them are not very technology literate) and this promises to do just that!!
However, in my quest to simplify WP as much as possible for my users I am already using the MEMBERS plug-in to create a custom role of "member" This is the default for accounts and is similar to author. In effect I only have two roles: Administrators and Members. This is very useful, especially when people post things they want restricting just to other members.
I realise that this is my own fault and so don't have expectations, but it may be that there is a more general need for your plug-in to support custom roles defined in the database?