I would be shocked if you were forced to go HR or Fin against your wishes. The way the new system works (new for the past 3-4 years or so) is that upon completion of Log Phase IV, you are employed in a Svc Bn in a number of junior officer positions (Pl Comd, AO, A/Adjt, Asst Ops O, Asst Trg O, Int O, couple others). The Svc Bn comprises a Tn Coy, a Maint Coy (Fd Wksp), a Sup Coy, an Adm Coy, and 2 x FSG Coys. Note that there is no Fin Coy or HR Coy (Admin Coy does not equate to Clerk Coy - it has a Sup Pl, Tn Pl, Maint Pl, and HQ)! You will in all likelihood work in a Supply or Transport Company, possible Admin Coy, and your platoon command tour will be of a Sup or Tn Platoon, regardless of which Company you work in.
After a year or two in the battalion, you will be asked which specialty you would like to pursue - Supply, Transport, Finance, or HR, and the battalion staff will make EVERY effort to slot you into the course you want. So, what you want to do when you get to the Bn is kick and scream to become either a Supply Officer or a Transport Officer - the CO/DCO and your OC will get the picture, and try to slot you on the course you want, and employ you appropriately.
After a tour in one of the functional companies, you will likely get moved to an FSG - those are the guys who deploy overseas.
Bear in mind that even if things don't work out for you the first time, and you get stuck on, let's say the Finance course, that doesn't mean you will spend the rest of your career as a finance officer - you can do another course at any time and get posted into a Sup or Tn job as well. But, like I said, it is highly unlikey that you would get slated into a course you dont' want - none of us Senior Log Offrs are trying to screw anyone around.
If you want to deploy, tell your Career Manager that you want to go to a Service Battalion - make a fuss about it. Once you're in the Battalion, show us what you're made of and you'll have ample opportunity to go overseas. Odds are, though, that you'll go nowhere until you've got Phase IV under your belt.