Well, at this point it is obvious you and I are trying for two separate things... (I'm trying to get the thousands of slaves we're fielding to not suck so much, your Phalangites are an entirely new unit that wouldn't do a thing to better spend all the excess manpower we're dumping into slaves due to running out of training... (Unless they end up using more than one manpower per unit, but our cavalry is the only unit that does that which doesn't need multiple men to operate) It's still a very good idea, though. Also my Levy suggestion might just be better served as a revision, due to no new tech or extensive changes to the base unit... So consider me leaning tentatively towards Phalangites for our design this turn.)
Either way, on to my suggestion for Take Three at rooting out corruption in the camps: (Not a design, for the Revision Phase.)
Personal Visits
Our commanders, or a delegate personally appointed by them, goes out and visits the camps where their future troops would come from at unscheduled intervals. As even the most cowardly, corrupt, or glory seeking commanders would want a well trained force (as such a force would increase their chances of not dying on the field of battle, or of claiming a glorious victory for the latter category), it is anticipated that they wouldn't be content with mere bribes to 'look the other way' when it comes to neglected soldiers.
When the time comes for soldiers to be sent to war, they will be put through a 'final exam', personally overseen by their future commander, to ensure that our trainers actually did their jobs in getting every soldier under their care up to par.
So, what do you all think? Inspiration struck and I had to get it down, which is why I didn't wait for the revision phase to post that bit.