I cannot say anything about the coding stuff. But if Expelliarmus would work like you suggested, it would lose most of it's canonic properties. Of course, Josh and Chessur went on my nerves, casting it while I was spell spaming, but Expelliarmus is as canon as possible, and that's pretty cool.
What I think is necessary, is a wand finding command. That wouod help in general, if someone lose the wand anywhere. It would make it possible (inbetween the time the drop doesn't disappear) to locate the wand (coords if it lay on the ground or IGN if it is in the inv of someone). But as far as I unerstand, I fear this woud the server make lagging...
There comes a point though here you need to draw a line between canonicity and what's best overall for the server. Sure it's pretty canonic at the moment, but consider that in a duel it's pretty OP and not to mention it can be used to steal a wand and burn it, which isn't cool.
If you still want it to be canonic, I have an idea. What if it was made so that instead of the wand being dropped at all, it is placed in the inventory of the caster? The spell would just move the wand around in the inventory of the victim like the spell currently does at low levels of the caster's inventory is full as obviously you aren't gonna catch their wand like a pop fly with your arms full. This wand cannot be dropped by the caster(solving the problem of burning) and will automatically be returned to the victim after x amount of time even if put into a chest(solving the problem of theft and being quite as OP). I think this would still keep it pretty canon, but also make it more fair and less cruel.