Caregiver informed consent (and participant assent) for research is common when working with populations with significant cognitive impairment. Human research review committees essentially always require that to protect participants.
You're certainly right that lots of people with early stage dementia are community living and not diagnosed.
>populations with significant cognitive impairment.
"Significant cognitive impairment" should not be characterized as, "mild dementia". If it is too difficult to recruit people with mild dementia for a study, then they should report their results as being relevent to only those with "moderate" dementia.
The diagnostic criteria for mild dementia are significant cognitive impairment that interferes with activities of everyday living. There are plenty of people still living at home with a caregiver/spouse, and people living at home on their own that meet these criteria.
You're certainly right that lots of people with early stage dementia are community living and not diagnosed.