If I remember right Thomas Bertilsson likes to uses Acros 100 at EI 400 for portraits. If he doesn't jump in here...PM him. Thomas uses X-tol also and his numbers may match above. If so, you're going to be in good company.
I've had luck pushing Across to EI 200 and developing in Xtol 1:1 for 14 minutes. This was in overcast lighting and is the most I would consider pushing the film without extreme developing techniques. I couldn't imagine the shadow detail i would lose in contrasty lighting. But for flat lighting it was great! Here's an example...
Based on admittedly limited experience, Diafine should also do a reasonable job for you. I've shot Acros at 250 and 320 and developed in Diafine, and the results weren't too bad at all. Diafine should help keep the contrast manageable.
As Michael said though, a test roll (under similar lighting if possible) is the safest bet.
I went ahead with 16 minutes, agitation every 5 minutes. The roll is junk, and development had nothing to do with it. I thought there was Tri-X in there, and I'm not averse to underexposing that by a stop if I need to, so Acros didn't stand a chance. There may be one or two salvageable images.
Moral: Don't underexpose your film by 2 stops+ stops.