My understanding is that the engineer would have to sign for the work under the approval of the M3 or Part 145 organisation i.a.w. their quality system, procedures, etc. If the engineer is doing it as a homer or favour outside of work then it is not a legal certification. His approval/authorisation is granted by the organisation, not the licence.
I may be wrong - I work on big aircraft, different rules apply to the wee ones. A BCAR licence with type rating may be legal.