Well this got me to thinking - I mean this is probably a serious hack. A report query in SSRS can almost be similar to a stored proc, that is, you can do multiple things before before you finally produce your result set.

So, a subscription is basically a SQL agent job identified by a GUID. There is an sp_update_schedule stored proc that would allow you to manipulate a job schedule. I think as long as the service executing the SSRS report had the required permissions, all of the pieces that you would need to update a subscription job schedule from report params are probably there. Not for the faint of heart - you have to understand the table schema of the ReportServer database and how to tie that to the SQL Agent job, etc.

Wouldn't be easy, but it might be possible.