I would not worry about this, at all.

The boiling point and volatility of acetamide and dimethylacetamide are far closer to that of xylene, compared to the boiling point and volatility of acetylated nmt.  Xylene and acetylated nmt are not going to form an azeotrope.  I haven't tested it, but I'm happy to bet my house on it.

On a side note, I have observed this procedure to work very well for separating nmt/dmt mixtures.  It really is very useful if you have acacias.  Good luck with it!