There's nothing forbidden about it but airmanship and common sense should preside here - if you are climbing at m0.8 in managed climb and get a negative v/s then go to open climb with selected speed and notch off a couple of decimals of mach. If need be take it back to green dot.
Hpbleed you don't ever want to get back towards green dot when climbing at altitude. Going open climb and then reducing Mach just exacerbates the problem, it gives you a quick zoom climb followed by speed getting low with low or negative v/s. You also have the danger of getting into ALT* with a high RoC, something which too many people are unaware or blasé.
To be fair the Easy 319/320s are not bad at climbing at altitude so you probably haven't seen this performance issue.