The leading question:
"Is it possible (and if so, how) to redistribute or originate a default route based on time of day?"
The short answer is "Sure, why not?"... But the longer answer has to do with how do we warp the forces of the universe to make that happen???
Well, start with what we know. We know we can do time-ranges in access-lists, right? Can we do them in standard access-lists (what we see used for redistribution all the time)?
Rack1R1(config)#access-list 1 permit 172.16.0.0 0.15.255.255 ?
log Log matches against this entry
Nope. There's a bummer. So we will need to use EXTENDED ACL's in order to make this work. So now we are reaching the point of "Yes, it can be done, but it will make my head hurt." as the answer. :)
First, as a little review, check out a blog we did last year providing some information on that sort of thing in conjunction with a distribute-list in different routing protocols.