In his commentary to that Numbers (20:8), Rav Sa'adyah Gaon renders "אל הסלע" as "על הסלע". (according to R. Yosef Qafih's Hebrew translation of R. Sa'adyah's Arabic.) In his notes there, R. Yosef Qafih explains that the intent was not that Moshe should speak to the stone, but rather that he should speak to the Jews in the proximity of the stone. Specifically, (R. Qafih explains in his notes to Rav Sa'adyah Gaon's explanation to verse 12), he was to speak to them about the infinity ability of God. The intent of the later verse (20:12) is not that Moshe did not believe in God, but that he did not inspire faith in God on the part of the Jews.
The Tanchuma (quoted by Rashi to verse 11), however, understand that Moshe was meant to talk to the rock. This does not seems so difficult as it is no less likely to cause water then hitting it.