According to
Wikipedia, one can combine arbitrary symbols with
diacritics and in my case, an
overline fits better than a macron.
At least it works in a message box:
MessageBox.Show("v" + char.ConvertFromUtf32(0x305));
Therefore, I consider the question closed.
My application needs it to work in a Windows Forms DataGridView. There, the overline is slightly shifted to the right (standard font and size). Since I have no incentive to twiddle around with don't-know-what-yet, I'll just use another symbol.
Thanks everyone for your effort.