Seura's vanishing TV mirror won't provide a bright mirror image for applying makeup.  It will be darker than a traditional mirror.  They have a non-vanishing mirror where the TV is visible as a dark rectangle when off, but this provides a bright traditional silvered mirror finish and the TV image looks better too.

You can get samples of the mirror glass to show your client.  I did a job a couple years ago with two large vanishing mirror TVs and ended up having to replace the glass on both to the non-vanishing version.

Client saw mirror samples before I ordered but wanted the cool "vanishing" TV... in the end, the mirrors just weren't good enough to use for makeup, etc.  Client even went through the expense to add additional lighting into the bath hoping it would help, but it didn't.

You can specify TV size and where to place the television.  Here's a photo after switching to the non-vanishing glass: