writing to SP Database is prohibited, reading is not recommended ;-) At the TechEd I've worked togheter with the Microsoft Support at the TLC booth. They told me that readind the DB is also not recommended, because reading could also cause an unwanted lock
in the SQL DB.
On the other hand, in the official course SharePoint 2007 course is one example accessing the SharePoint DB with reporting services.
The perfect way in SharePoint 2007 would be a custom SharePoint Designer Action using the Object Model.
The agile way would be scripting the Object Model with my PowerShell Action for SharePoint Designer.
In general the Execute SQL should support all ADO Data Provider, but I never tried this scenarios...but sounds interesting.
In SP2010 this could be a scenario for BCS.
But, actually I'm not sure if I understand your scenario.