I do not know what the minimum temp drop for cornsnakes is to make a biological difference and hence to legitimately call it brumation. That being said I have known of tons of people who successfully breed corns on a very regular basis without brumating or droping temperatures manually at all. So while it might not qualify as brumating it will be adequate to get baby corns in general. You just might get fewer then you would have gotten if you had actually brumated them. i'm sure Don and others who are experts on corns can give much more specific information.
That being said you do need to monitor body weight because what Herpzilla said about weight loss at the higher temps is true. I would be ready to resume feeding if they start losing much weight.
I know with Brazillian Rainbow Boas, which are normally kept at 82degrees, you can brumate them by droping down only 12 degrees to 70 but that if you do so you must continue to feed them througout the cool period. Although granted the feedings may be less frequent then the rest of the year or slightly smaller food items.
From what I've heard in the past with corns it is not worth attempting to brumate unless you can for sure get the temps down at least to the 60's. Especially since they can breed successfully without brumation and if not brumated the female continues to put on weight during that time which helps with egg production.
Hope was helpfull and I'm sure one of the experts will chime in with better more precise information soon.
Sean
-----
1.1 BRB
1.1 Triple Het TPRS's
0.1 Silver TPRS
1.1 Amel Bloodred Corns
0.1 Abbott Okeetee Corn
0.1 Blizzard Bloodred Corn
1.1 Thayeri Kingsnakes
0.1 Reeve's Turtle
0.2 Amstaff's
1.0 Pudytat