Yes.

Can he? Yes Can he be BO and contribute to OH time and speed? No

For PBB yes.

Use a ENG. I tried with a REP and sight lines are too short. 

The answer is yes.

But if you plan on using an engi as BO for your PBB you will find the guidelines are rediculously long (something I only just learned myself) - which makes it quite difficult to measure your range.