No. If you asking who in particular has a particular job or function or name, then it must be ydy. If you are asking who in general is a whatever, then it is sy:

• *Pwy ydy'r technegydd? - Who is the technician?

• *Pwy sy'n dechnegydd? - Who is a technician?

• Pwy ydy Dewi Lingo? - Who is Dewi Lingo?

• Pwy sy'n dysgu Cymraeg yma? Who is learning/teaching Welsh here?

The sentence you suggest would actually be "Who is in the technician?".