Thanks for the reply.
Once you enter a manual speed on the VNAV descent page, like .85/300, you have just entered the first speed constraint and the DRAG REQUIRED message will now be displayed once you reach 310 KIAS.
This is an interesting theory and would certainly explain the DRAG REQUIRED displaying well prior to 349 knots. But why wouldn't it also apply to prevent the
speed from going up to 354 knots. Once below the first speed constraint (as you said entering .85/300 would put you in that position on descent), the maximum speed to maintain the path would then be 15 knots above FMC target speed (page 2). In this case, if you entered .85/300, wouldn't it only go as fast as 315 knots to maintain the path before speed reversion occurs? Would there even be a need to speed intervene at Tod as you mention? I see that the second scenario on page 2 (after you have passed the first speed constraint) doesn't include a maximum mach. Are you suggesting that 315 knots would overspeed the mach?
If that's the case, it would be one of the rare cases where VNAV would not provide mach overspeed protection.