Adebrophy - hmmm finding a room with a window in a Japanese office might be difficult!
(This might not give you "professional" arty portraits, but will give properly exposed headshots; however it is so stupid lighting that you may not even think it is worth it)
Ok, can you find an inside corner of two white walls? Then stand kind of at the vortex of the two white walls such that the two walls are behind you (imagine standing in the middle of "V") Ask your subject to stand at the open end of that "V", mount the speedlight on your camera but facing backwards towards the corner and fire. The results always pleases me for simple headshots.
Another good headshot lighting is available in hallways (not too wide and with white walls). Put the speedlight off camera facing one of the walls (right or left) Now depending on where you make your subject stand you can get differential lighting on both sides of the face. You may also use two speedlights hitting both the walls and get more even (or by chaning power of the flashes differential) lighting on both sides of the flash. For the headshots I will always go for bounced light (assuming that you do not have softbox or any such contraption). And if possible get some large white chart paper (may be called art paper or cartridge paper in your country). You can probably pin or tape them on any wall or glass window or partition or even white board and use them as reflector for any of the above lighting set ups.
Also pay attention to how you are asking your subjects to pose for the headshots. Do not let them stand as if you are taking passport photos. Make them stand side ways facing you. They may rest their hands on something, brace themselves tightly or looses.... different poses with similar lighting will also give you different results.