I've never really thought about the combo, and even after some examination, I really can't see the complete point to it. Beastmaster as a sub is a mixed bag, as people often can't agree on how taming is done. From my experience it seems as if it's a combination of your main classes level and your beastmasters level, along with your charisma. If you're a high Cha race, slap on alot of +cha items, and keep your subjob maxed out, you might be able to charm an easy prey after 1 or two tries, with the charm lasting as long as a normal charm from bst as your main. However, if you tried taking evens or high decents, I think you're going to have an exceptionally hard time getting the charm to stick, maybe 3-4 tries on average.
Also, with BST as sub, you're going to have to wait until level 70 before you can release your pet from battle, which is THE reason why beastmaster is so very good after 35. the str and hp you get from a bst sub won't be atrocious, so you're not going to be hurting that much in terms of stats, but I can't really see you using your pets all that much, as you won't be able to release them and make up for that 30% xp loss. Heck, you won't even be able to use return to kill your pet off until level 20, that should hurt alot if you're solo'ing.
If you're going to group, you might be okay, but I wouldn't suggest taking a pet, as you (usually) can't find easy prey in zones where you're fighting (of course there are exceptions), you won't be able to release it and get your full xp, and because it's a lower level pet, the amount it helps in the battle will be negligable. Not to mention the fact that many groups want monks with a war sub for emergency provokes, so you're going to be less desired.
If you can live with all this though, i'd be delighted to know how it turns out.
Class(es): lvl60 Hunter/Mage/Druid
Server: Sylph Character: Grelix
1win-2losses on Maat ^^