Is it possible for a research assistant to be my subject-specific mentor?
No, unfortunately this is not possible. The subject-specific mentor must be a tenured professor. This also includes junior or deputy professors. Guest professors, honorary professors or senior professors are not included. The reason for this rule is that you can - if necessary - get better support from a tenured professor in organizational matters (e.g. room booking, credit transfer).
Please note: You can, of course, obtain additional supervision from other persons (e.g. research assistants). You are welcome to include this additional supervision in your project outline.