Form API autocomplete

Hi,
I'm trying to add an autocomplete form field via form API.
Usually Entity Reference Autocomplete works quite well.
But it does not work for CRM core contacts because it is trying to filter by label. Since contacts don't declare a label it does not work.
Usually the label is a column on the entity, but since we usually filter by contact name this is not possible since the name is a field stored in a different table.
So my first idea was to create a label field that is always updated on entity save to copy the name information to that field.
Then the entity could be filtered by label ( name ).

What do you think about this? Any other ideas for a better approach? Maybe there is even a possibility to have an autocomplete reference field via form api that i didn't see.