So, a desperate suicide move to lock Ichigo into whatever it is and win some time? (since he definitely won't be able to lock him forever. Ichigo always has an option of going into deep meditation for months, and restoring his previous abilities. Over time, he'd be able to figure out not just how to get out of that cage, but out of the dimension as well, I guess.
Anyway. I doubt that one arrow is enough to mortally wound Urahara, aaand... that bankai-stealing metallic plate is in his hands as well. Which means, his enemy cannot steal his bankai
Crossing my fingers to see Urahara's bankai in nearest future.