Go for it. You'd be amazed at the places you could take a stock X5 on "snow tires". I know 2 or 3 people who got in places they shouldn't have been and made it out while more traditional rigs got stuck. With the right setup, it should handle most Expo duty just fine. The only downside is the probability of damage on the trail and the lack of easy parts (both in the field for repair and for aftermarket customization.) Having custom skidplates made (from aluminum) would be a priority.
As far as rims, might be worth keeping a set for off-road use and a set for normal driving. Chevy used the same bolt pattern on 2nd generation S10s and Blazers (95-03ish) but had a slightly smaller hub bore (72.6mm for BMW, 70.3mm for Chevy.) They might be able to be milled to fit properly. 4wd models had about 5.5" to 6" of backspace while 2wd models were 3.75. Not sure if the BMW uses a lug or hub centric setup, the Chevy ones are hub centric.
If it were me, I'd do this: You could look into a take-off set of e36 M3 5-spoke rims. They're light, very rugged, can be had relatively cheap ($200 ea, refinished) and fit correctly in either a 17x7.5" or 17x8.5". If I remember correctly, these are also forged (not cast) so they're much stronger than most aftermarket options.
Tires in 235/65-17 (29" dia) and 255/55-18 (29" dia) can be had and are stock sizes. In 235/65-17 you can use the Yokohama Geolander A/T-S or the Pirelli Scorpion ATR. In 255/55-18 you can use the Pirelli Scorpion ATR, Bridgestone Revo 2 or General Grabber AT2 (best bet.)
BFG offers their new Rugged Terrain in a 235/70-17 (30" dia) and a 245-65/17 (29.5" dia.) They might fit. I just put a set on an F250 and they ride amazingly smooth yet look to be aggressive enough for most Expo use. Definitely more aggressive than the other AT/HT hybrids like the Revo 2, Geolander AT-S and Dynapro ATM. They look more aggressive in person than in the pictures, at least the 275/70-18 ones do.