I think it would be okay. Most people use their engagement photos to do their trials. I am going to do my trial for a Formal Fundraiser dinner for my FHs College that he teaches at. He will not see the veil on but he will see my hair and makeup. I don’t think it will spoil too much especially if he hasn’t seen your dress.
I did my trials for my bridal shower so my wife, and everyone else, saw them. My hair stylist styled my hair so that I could see the look that I selected and then changed it so it would be slightly different for the shower. My MUA did something similar by changing my lip color and toning down my eyeshadow.
I did my make-up trial the day of our engagement shoot. I wasn't worried about him seeing me, because I know that he would not remember my make-up months later. I don't want him to see my hair before the wedding though.
My FH saw mine and loved it...but he won't see the dress until day of. Sometimes it's nice to give a preview of something so they realize how special you are making yourself look and feel for when you walk to them down the aisle.
I did my hair and makeup trials on different days but my husband saw me both times! I used the makeup trial for a date night I figured that it wouldn't be nearly the same as him seeing me in my dress with the full getup!