Is it possible to retrieve the specific databases selected in a maintenance plan step?
I don't believe there is a way.  If you create a Maintenance Plan, it is going to be an SSIS package.  Querying the msdb.dbo.sysjobsteps table, you can view the actual command, but for an SSIS package execution it won't be indicative to the particulare databases you are backing up:

select
    step_name,
    subsystem,
    command
from msdb..sysjobsteps


This will give you something similar to the following command:

/Server "$(ESCAPE_NONE(SRVR))" /SQL "Maintenance Plans\MaintenancePlan" /set "\Package\BackupDBs.Disable;false"