Perhaps try using a small-sized longspear as a medium character? A 2h small weapon counts as 1h for a medium creature. Of course, that kind of invalidates the entire reason you went Phalanx Fighter . . .
For what purpose are you trying to one with one free hand?
If it's for magus then the Skirner magus and Phalanx fighter mesh well and using a buckler would be more than appropriate. This has the bonus of full armor and shields as well as no problems casting (due to the skirner's abilities). Down side? You are behind 3 levels on casting and you don't get actual spell combat until 11th level~ish.
Another option is to use martial versatility to adjust off of a sibat onto the fauchard (taking slashing grace with the sibat).
Isn't that usually frowned upon by gm's
Well after you factor in the -2 for inappropriate weapon size and the fact you won't gain any of the usual 2H bonuses with it it's not as bad as it sounds. Or you could just grab a whip and Indiana jones it.
When you use a weapon as something that is what it is.
Notice that if you have martial proficiency with bastard sword you cannot use it as an one-handed weapon at all (even at the non-proficiency -4 penalty) because it is a two handed weapon. If you have the exotic proficiency with bastard sword though it is a one-handed weapon and suddenly you can do all the things with it that you couldn't do before (including using a larger one two handed with a -2 penalty).
This is a mantra repeated throughout the weapons rules -- how you use it is the type of weapon it is.