Depends on whether the Empire can get in, or even knows where he is.
If they can't, incident ensues, Palpatine doesn't play by spell-card rules, someone might get killed (POWAAAAAH UNLIMITED POWAAAAAH) and then Reimu or someone kills him.
If they CAN, then good luck. Only Flandre (if she's sane enough to destroy all the Empire) or Yukari's boundaryhax can save them.
What if Gensokyo was set in China?