So, just out of curiosity, I went and did the maths with a default Mega Blaziken set. Since HP never changes, the only stat I actually need to track is DEF. Modifying to include a Grass/Dragon typing and giving Mega Sceptile a bog-standard bulky tank EV distribution (max HP, max Attack or Special Attack), you need to increase DEF by 19 before you turn the OHKO into a 2KO on Flare Blitz.
252+ Atk Mega Blaziken Flare Blitz vs. 252 HP / 0 Def Mega Sceptile: 357-421 (103.7 - 122.3%) -- guaranteed OHKO (+0 base DEF)
252+ Atk Mega Blaziken Flare Blitz vs. 252 HP / 0 Def Mega Sceptile: 291-343 (84.5 - 99.7%) -- guaranteed 2HKO (+19 base DEF)
It's not entirely beyond the realm of possibility, but the issue is that Sceptile doesn't really have anything going for it other than the neutral Dragon STAB to counter Blaziken. Short of a STAB max-invested, Modest Draco Meteor (which move, I might add, Sceptile doesn't presently learn, nor does the other Mega that gained a Dragon secondary type, Ampharos - it's completely limited to actual 100% Dragons and Arceus), it can't manage to get the OHKO reliably without significant increase in Special Attack, and it's highly unlikely to outspeed a Speed Boost Mega Blaziken to score the 2HKO first (especially since, remember, these are bulky EV sets, so there is no speed investment).
Finally, it's useful to remember that this is checking Flare Blitz, since that's Blaziken's most powerful STAB, but it entirely ignores the super-effective Brave Bird. To avoid the OHKO with a Brave Bird from said Adamant Blaziken on a base 70 HP, you need 119 base DEF, or an increase of 54. As Rekka notes, Megas tend to only change base stats slightly, relatively speaking; such a one-sided increase would be unprecedented.
EDIT: No, actually, it wouldn't be unprecedented now that we have Diancie. But still, it would be rather unusual.