It would depend on what the atmo engine regs were - 6R4's are atmo
The change to atmo in rallying flies in the face of the proposed changes to F1 regs (these are now talking about 'energy recovery systems' or 'Turbo' in plain speak). The proposed atmo regs in the other thread appear to have vanished without trace (good digging to find that old thread!), I guess S2000 has superseded these to some extent? WRC's are limited on power output anyway so why the need to change the engines? Especially if the power output from the atmos would be the same.
Even if rallying did go down that path then there's no reason for rallycross to follow - how many components are common in a WRC and Div1 engine anyway? Not that many - we've seen the difference demonstrated this year with George Traceys 307. Rallycross needs to keep the performance of the Div1 cars well above that of WRC's or their future equivalent in rallying.