Not forbidden. I attend both RC and EC parishes. I do my best to follow the customs of the church that I am in. When I serve Mass in the West I have been asked why I cross myself “backwards” or why I cross myself at certain times when there is mention of the Holy Trinity? Those are honest lapses in concentration, but you do have to be careful or some people will begin to mimic you.
In the East the default location for our hands is not “5-on-5” as it is in the West. It’s simply hands at our sides. Many times I would make the mistake of assuming the “5-on-5” and other acolytes would begin to copy me or my “backwards” RC crossing, so I had to focus more and remember where I am.