Strictly speaking, if the informal arrangement doesn't comply with terms in the Marital Settlement Agreement, then no, it's not legally binding. However, practically speaking, whether or not a judge will enforce an informal arrangement depends on the judge, and there may be other factors (not included in your question) that would affect a judge's ruling.
Informal communications (i.e., email or text) can be used as evidence of an agreement, but there's no way to accurately predict how much weight any given judge would give this evidence, or whether the judge would accept this kind of arrangement. There are no guarantees once you step outside of the MSA.
It's always best to comply with the terms spelled out in the MSA, since these are the terms the parties have already agreed upon. If the MSA requires a writing signed by both parties for an agreement to be binding, the implicit warning is that the court has no duty to enforce any arrangements made otherwise.