How to add hair on faces hard to reach?

[TanzaniteMNK (Yukari)]

So I recently found out that I can’t add hair on a mesh that is posed or has a shape key applied. This made me wonder, is there a way you hide certain faces so you can edit or add hair on another face?

[Ruwo (Rudi Wohland)]

One possibility is to use the mask modifier. First you have to define a vertex group and then apply a mask modifier and select the vertex group in it.