Having the engines mounted forward of the wing flexural axis won't affect aileron reversal.
Aileron reversal can be either purely aerodynamic, or it can be aeroelastic.
If the former, it occurs because the ailerons are disrupting the flow in some fashion that causes the local lift to behave in the 'wrong' fashion. An example of this generally occurs at low speed/near the stall, where deflecting the ailerons downwards causes the local flow to separate, and thus the lift reduces instead of increasing, and the aileron operates opposit to the desired sense.
If the latter, the wing stiffness is too low and the torsion induced by the aileron causes the wing to bend, as mentioned by pigs, and if the resulting change in local AoA is more powerful than the trailing edge deflection then the aileron reverses in effect. But the mass distribution on the wing doesn't matter - it's purely driven by stiffness in this case.
re other suggestions: cg isn't a player, if the wings aren't carrying the engines then the basic design allows for that - aft-mounted engine aircraft don't sit on their tails.
giorgino's answer is basically the same as mine except I called it aeroelasticity and he called it flutter, which are two names for similar things. (flutter is basically divergent dynamic aeroelasticity)